For the growing number of Windows Phone users, Windows Phone 8 was a frustrating release. The major difference between Windows Phone 7.5 and Windows Phone 8 was invisible to end users: merely a kernel swap, going from Windows CE to Windows NT.

Strategically, this was tremendously important for Microsoft. The company is on a trajectory to have a common operating system core across phones, tablets, desktops, and TVs (with the Xbox One console), enabling developers to have substantially the same code running across all these different systems.

But being strategically important doesn't really matter a whole lot to end users. As we noted at the time, Windows Phone 8 was a solid and usable smartphone platform, but it lacked any big headline features. It made lots of things a bit better, but didn't do anything to really convince people to give the platform a second look.

Moreover, Windows Phone 8 was but a stepping stone toward this future vision. Although it offered a few APIs in common with Windows 8—and there were techniques for sharing code between the two—for the most part, developers had to write two substantially different applications if they wanted to run on both platforms. Back then, Microsoft wasn't really talking much about the future, but even so it was clear that Windows Phone 8 wasn't ready to be a part of it.

While iOS and Android remain dominant in the smartphone space, a few things have changed since the release of Windows Phone 8. To start, Microsoft's operating system has started to carve out a niche for itself with a market share above 10 percent in a number of markets.

This success has often been on the back of low-end phones. Nokia's great value Lumia 520, for example, has proven popular, offering a full smartphone experience on a device that sells sometimes for as little as $60 without a contract. Windows Phone has proven to be consistently robust even on low-end hardware, with good performance and features ensuring the user experience remains solid even when the hardware is highly affordable.

Conversely, the experience on high-end devices with large, high-resolution screens was quite lackluster. Here, Microsoft offered an inconsistent approach that sometimes allowed apps to take advantage of the extra screen real estate, but most of the time left them comically large.

Windows Phone 8.1, therefore, has a lot of work to do. It needs to take further steps along the path toward Microsoft's vision of a unified operating system. It needs to work better on a wider range of hardware to both strengthen its position at the low end and give it a chance of making inroads at the high end. It needs to also offer features: it needs to do things to get people talking about the platform while attracting both users and developers.

Remarkably, Windows Phone 8.1 delivers on all fronts.

A ton of new features

After the Windows Phone 8 release that didn't really add any big user-facing features, it's about time that Microsoft had a real feature release. Windows Phone, while livable day-to-day, still has gaps compared to iOS and Android. And while it does have some striking features (most notably its eye-catching appearance), it hasn't really had anything to make people who don't follow smartphones or Microsoft take notice.

Cor blimey, it's Cortana

If anything will get people talking about and taking notice of Windows Phone, it's Cortana. Cortana is a name that will be familiar to Halo players (which I am not). In those games, she is an AI assistant that aids the protagonist in his quest to kill aliens, or whatever it is he does. Naming it after a thing in Halo is cute enough (though, spoilers, Cortana apparently dies in Halo 4), but in practice, it's not a name that means a whole lot to most people. Microsoft can't count on immediate recognition when people hear about it. (Personally, I would have called it Holly.)

Microsoft describes Cortana as a "more personal personal assistant." What is she actually? Cortana is a combination of Siri's smart, contextual, voice-driven (or text-driven; anything you can say to Cortana you can also type to her) interactive helper and Google Now's proactive, data- and location-driven assistant.

This means that Cortana can do Siri-like things—create appointments, set reminders, send messages, find local restaurants that have at least four stars but which won't break the bank—while also doing Google Now-type things—tell you when you need to leave for your next appointment given current traffic, warn you that your flight is late, show you the exchange rate for your trip abroad.

Cortana's Little Red Book

The big difference between Cortana and comparable features on other platforms is the setup and control. The first time you use her—when enabled, she replaces the Bing search invoked from the search button—you're asked a few questions about your interests, and you give her permission to look at your data. You don't have to let her look at your e-mails if you don't want to, but features like flight tracking depend on it.

All this information gets stored in the Notebook, where Cortana is controlled and configured. Interests can be added and removed, and some of them offer finer control. For example, if "traffic" is one of your interests, you can choose to have active alerts whenever you need to leave your current location to reach a meeting, given the prevailing conditions. Some, though not all, of the interests can also be pinned directly to the Start screen.

The Notebook also lets you see all the reminders you've set. Cortana's reminders are pretty neat. As well as the obvious temporal reminders, she has location-based and person-based reminders. With these, Cortana can remind you to do something when you go somewhere or when you talk to someone. For example, you can ask for a reminder to buy Coca Cola Vanilla Zero (highly recommended) next time you're at the store or for Cortana to tell you to ask your sister about her new kitten next time you call her.

It's a bit weird that she pronounces H-E-B as "Hebrew" but only does it once.

Cortana also drives a "quiet hours" feature of the kind we've seen on other platforms. Again configured through the Notebook, she can automatically suppress notifications at certain times of day or when you're busy in meetings. Your "inner circle" of contacts is allowed to break through quiet hours.

The Notebook seems like a sensible way of keeping Cortana on-topic and focused. Last year we asked Microsoft's Bing team why they didn't yet have a system comparable to Google Now, and they said that creating a system with this kind of control was one of the things they wanted to do before deploying such a feature.

With Google Now, if you perform a few searches to find out results for a particular sports team, Google figures out you probably care about that team's performance. Google Now will start to show those results without asking. Cortana does know about sports teams and lets a user manually specify teams as "interests," but she doesn't seem able to infer that if searching for a team regularly, the user probably cares about them.

Thanks to the Notebook, Cortana could, and should, be similarly aggressive about learning about a user. The Notebook provides a good system for fixing mistakes—you could be searching for a team to answer a friend's question and not actually care—so it would make sense for Cortana to be a little more speculative. That is, after all, one of the things that real personal assistants do: they figure out what you like and dislike without having to be explicitly told.

There are also a few things that, surprisingly, Cortana doesn't seem to have as interests. For instance, you can search for stock ticker symbols, and Cortana will give nice structured results when you do, but there doesn't appear to be any capability to remember that you follow certain stocks and tell you about them automatically.

Journalists who work from home aren't the best Cortana users

In testing, we found Cortana's speech recognition was surprisingly consistent. My British English accent and reasonably clear enunciation generally posed no problems for Microsoft's speech-to-text processing. We didn't really have an opportunity to give Cortana a real-world test (the Ars Orbital HQ doesn't always involve a lot of running around), but using her for setting reminders, creating appointments, and so on and so forth, worked correctly for the most part. Her interactions are reasonably fluent, and she seems able to do a respectable number of tasks.

(And if you feel like a jerk talking to a computer, you're in good company. But you can type to her, too, and she'll work the same way).

Cortana's capabilities will grow in the future, too, as there are various third-party extension points enabling apps to integrate with Windows Phone's speech capabilities. Prior versions had limited speech extension points, but built around canned phrases rather than arbitrary speech-to-text processing. It's not immediately clear how effective this extensibility will be, and no apps seem to be able to take advantage of it yet.

Still, consider your day-to-day before jumping to a phone solely for the assistant. The unfortunate fact is that my lifestyle is not conducive to testing this kind of software. I work from home, I sit in front of a computer most of the day, and I don't drive. I recognize that Cortana can do lots of clever things, but my habits don't really give it the opportunity to shine.

This isn't meant as a knock against Cortana; I had a similar experience when using Google Now. Google Now didn't do a whole lot for me—mainly just show me the local weather, as Cortana does. But when I had a trip abroad, it sprung into life with flight info, currency details, reminders to me to leave for my appointments, and so on. The change in circumstances let it prove its worth.

While the speech and interaction capabilities are the ones that immediately turn heads, it's the proactive capabilities that are more interesting. While I can't use Google Now all the time due to my lifestyle, I've found that when I do use it, it's much more useful and less gimmicky than Siri. Cortana's capable of doing all the right things, the things that I would expect this kind of intelligent agent to be able to do. I'm confident that given the right circumstances, Cortana would prove her worth to me—I just haven't yet seen it, which is my fault more than it is hers.

Powered by Bing, still in Beta

Behind the scenes, Cortana is substantially powered by Bing. It's combined with a few new platform features such as geofencing, which alerts apps whenever you reach a certain location. There's a lot of technology involved, and it's going to need real-world testing, especially for things like the conversational speech recognition, before Microsoft is willing to call it "stable."

As such, just as was the case with Siri, Cortana is going to ship initially with a "beta" label. She will be available only in very limited markets, with the US being the first. In spite of the beta label, we didn't find anything egregiously wrong with the system and hope the rollout happens sooner rather than later.

If you're in a non-Cortana market—or if you elect not to enable her at all—the system sticks with the old Bing search app. This has been enhanced a little. As well as the local, images, videos, and Web results of the old version, there's a new "phone" context, which will find e-mails, calendar items, apps, contacts, music, and text messages. (Cortana-based searches work similarly; generally, if Bing can do something, Cortana can do it too.)

The right kind of feature for showing off

The biggest thing that Cortana gives Windows Phone is a flashy feature to show off. Surely she will prove useful, but that's almost beside the point; like Siri before her, Cortana makes great demoware. She's equipped with various jokes and easter eggs. Ask her to open the pod bay doors, and she'll tell you she's sorry, Dave, she's afraid she can't do that. Tell her to sing a song, and she'll regale you with Daisy, Daisy.

I don't think she's going to win America's Got Talent.

While her voice is mostly synthetic, there are also canned responses that were recorded by Jen Taylor, the voice actor who provided her voice in Halo. She was familiar to me as the voice of Windrunner, Lina, Puck, and Medusa.

Imbuing her with this kind of personality only makes her more valuable as a promotional tool. Even though it'll be some weeks before anyone other than developers can use her, she's already been featured on The Arsenio Hall Show twice. An extended skit where she argues with Siri has garnered a bunch of YouTube views.

The only thing that might temper that somewhat is a sense that perhaps we've seen this all before. As Arsenio made clear, this really is "Microsoft's version of Siri." That could prove to be a curse, but Cortana has the chance to be a lot more useful than Siri. The direct comparison actually does a disservice to Microsoft's technology.