The four-star is the best Windows Phone yet, and the best chance so far for the Windows Phone platform to finally take off in the market. But the challenge Microsoft faces was starkly illustrated for me when I handed my wife a Windows Phone and she promptly asked, "Can I play Draw Something?" Draw Something hit 50 million downloads today, but not a single one of them was on a Windows Phone. It just isn't available.

That's not say there aren't plenty of Windows Phone 7 apps to choose from. In fact, there are now more than 70,000 apps in the Windows Phone Marketplace. "We offer 90 percent of the apps identified as most popular by customers on both leading competing platforms," Microsoft said in an email to me, and I agree; many of the apps on "top apps" lists for any platform are already available on Windows Phone. But there are a few key omissions, including some critical social games that are lighting up Android phones and iPhones.

Fortunately, the company seems to be on the case. When I sent it the follow list of missing apps, Microsoft responded, "We are talking with the developer behind every app on your list … it's a matter of when and how. … In the meantime, most customers find that between our 70,000 apps and great IE9 Web browser, there are very few services or scenarios that aren't available to them on a Windows Phone."

Regarding casual games, Microsoft said, "We'd love to have all of the most popular game titles on our platform, and, again, no one on your list has told us no. … Microsoft invested heavily in premium games through Xbox, whereas Android and Apple typically have less sophisticated casual games higher up the most popular list—that was a choice. Our Xbox LIVE portfolio for Windows Phone consists of 108 games, 16 of which are exclusive to Windows Phone."

I took six of the apps on my top-10 list from Nielsen's list of most-frequently-downloaded Android apps and Apple's list of all-time most-popular iPhone apps, so we know they're popular. Of the other four, Audible and Nook make a point about cross-platform content; Instagram and Temple Run have huge user bases. I've highlighted why each one is important below.

There are other popular apps missing from the Windows Phone platform: MLB.com, Mint.com and HBO GO come to mind. Windows Phone needs those apps, too, but each of them is less important than the ones on this list. The apps on this list underscore the social effects which lead a new smartphone owner to consider a platform "safe" to buy. People see gadgets and technology as "safe" when a lot of people they know have them, use them, and don't have serious complaints. Windows Phone 7 doesn't have this luxury yet.

Microsoft has shown that it's willing to put money on the table to make sure Windows Phone owners have the right apps in important categories, and in its email to me, the company said it was aware it still had work to do to attract "the top apps that people use every day." That's certainly the right attitude, the question is, how quickly will it happen? To grab the casual and social smartphone owners that are Windows Phone's prime targets, Microsoft has to get a handle on these ten apps now.

10 Apps Windows Phone 7 Needs to Succeed

Angry Birds Seasons

Windows Phone is missing huge parts of the multi-million-selling Angry Birds experience. While the platform has the regular Angry Birds, it lacks the popular spinoffs Seasons and Rio, which have tens of millions of players each. Angry Birds is a sort of canary in the coalmine for the viability of a mobile platform, and with nearly every smartphone on the planet running Angry Birds, the missing Seasons and Rio really make Windows Phone look like a laggard.

Audible.com

Audible is a popular, cross-platform audiobook app. Having Audible apps on multiple platforms lets book owners leverage their purchases and not lose money when they move OSes. The lack of an Audible app on Windows Phone prevents audiobook listeners from moving to the platform. Fortunately, it looks like this app may be coming soon.

Cut the Rope

I have a lot of casual games on this list because they have mass appeal with download numbers in the millions. Casual games also appeal to new smartphone users, who are Windows Phone's main target. And Windows Phone is doing pretty well on "gamers' games" thanks to its Xbox connections. Cut the Rope is another major casual game which your iPhone and Android-packing friends may be playing, but you won't be able to.

Draw Something

Casual smartphone owners love social games, and Draw Something is the hottest one out there right now. If all your friends are playing Draw Something and you can't, then you're a bit of a social outcast, and nobody wants that - especially on a platform that prides itself on social features. Zynga, Draw Something's owner, refuses to develop for Windows Phone. Microsoft needs to make it rain chez Zynga until the company starts writing some code.

Instagram

I hate Instagram, but it's hugely, thunderingly, massively popular on iPhone and Android, and I can't help but think that its presence on Windows Phone would give iPhone owners some comfort if they're switching. There are Windows Phone apps which do similar things to Instagram, but brand matters.

Nook

Like Audible, Nook is a cross-platform bookstore, and its presence lets people with a lot of locked-down book purchases move to the platform without worries. Windows Phone has Kindle, but the platform can't afford to shut out the millions of Nook (and Nook Color, and Nook Tablet) owners who have invested in Barnes & Noble content.

Pandora

More a bellwether than anything else, Pandora is a hugely popular app on other platforms and the Web. Windows Phone owners have Slacker, which does pretty much the same thing. But "it doesn't have Pandora" is a cheap shibboleth for "prominent developers don't want to create apps for the platform," and Windows Phone needs to shake off that stigma.

) now has more than 45 million players, according to the developer. As Imagini Studios says, "All your friends are playing it." I consider Temple Run to be a great example of a fad app which gets huge water-cooler buzz. Who wants to be the only person in your peer group who can't download it?

Viber

Viber is a popular VoIP/IM app on iPhone and Android, and it's no. 6 on Apple's list of most popular iPhone apps ever. Windows Phone owners need to be confident that their friends on other platforms can call them using their favorite app. Apps like this have strong network effects; if your friends are on Viber, a lack of Viber could kill a phone sale.

Words With Friends

Just like Draw Something, the Scrabble-like Words With Friends is a global social phenomenon. The game binds together affinity groups just like Facebook and Twitter do; it's something that your friends may assume you have, where you'll feel like you're on the outs if you don't. And sorry, Words By Post won't do if all of your friends are playing Words With Friends. This is another Zynga game, so if Microsoft cracks the Zynga nut, it could bring some word fans on board.