When Microsoft set out to build Windows 10, the company had a challenge to face: the operating system needed to appeal to the wide swath of people already using Windows.

"We joke on our team that we're ordering pizza for one-and-a-half billion people," Mohammed Samji, a principal group program manager at Microsoft, said in an interview.

To serve all of those users, Microsoft wanted Windows 10 to work well for people no matter what interface they chose. Windows 8 was criticized by those who said that Microsoft had moved around too much functionality in order to accommodate tablet users. The company took back some of the changes with its release of Windows 8.1, but this new update is designed to appeal further to traditional keyboard and mouse users alongside owners of touch devices.

To facilitate that, Microsoft has been working with outside stakeholders on the operating system since January 2014, when the company approached some of its enterprise customers to discuss Windows 10 under the cover of nondisclosure agreements. Since then, the company opened the Windows Insider Program, which allowed anyone to pick up a pre-release version of the operating system for testing purposes. Feedback from those user translated into tweaks to Windows 10.

"We've actually changed the plan," Samji said. "We've added things, we've removed things based on the feedback from our 5 million best friends that are actually testing the product for us."

There are now 5 million Windows Insiders, and their feedback helped shape the operating system's direction. Thanks to changes made based on Insider feedback, the operating system features more translucent elements similar to Windows 7's Aero Glass, and Microsoft's new Edge browser includes a dark theme.

Overall, Samji said that Windows 10 should feature something for everyone, including an interface that works well whether people are using a mouse and keyboard, a touchscreen, or some combination of the two. The operating system's interface changes itself for different modes of interaction, which is important for increasingly popular 2-in-1 devices that can behave like both a laptop or a tablet.

Reviews of Windows 10 appear mostly favorable thus far, which is a positive sign for Microsoft's initial efforts.

Of course, it remains to be seen whether users will take to the new operating system. Microsoft is hoping that there will be 1 billion devices running Windows 10 within three years. Wednesday marks the first day the operating system is officially available for consumers, and it will be interesting to see how many of them hop on board.