After a trying first year in the market, Windows Phone is back for a second go-round in the form of Windows Phone 7.5. Previously code-named Mango, and available as a free upgrade for all current Windows users, this new release expands on the broad themes Microsoft pioneered with the initial release and fills in almost all the functional gaps that had early adopters griping.

But I know what you're thinking. Is Windows Phone 7.5 enough to establish Microsoft's OS as the third viable mobile ecosystem alongside Google Android and Apple iOS?

I can't speak to buying habits and unit sales expectations. But from technical and usability perspectives, the answer is a resounding yes. Windows Phone 7.5, like its predecessor, offers a compelling alternative to the tired UIs Apple and Google are flogging their own users with, and is the superior mobile OS.

The key to this success can be found in a neat phrase that Microsoft is apparently trademarking: "Put People First." But this isn't just marketing drivel. Windows Phone really does put people first, and it does so in ways that its competitors most decidedly do not. It is, in the words of Microsoft's Greg Sullivan, a huge differentiator for the platform.

"Windows Phone puts people first," Sullivan told me in a briefing Monday. "It works the way you do, and doesn't force you to work the way it does." The canonical example of this behavior is photos. When you want to view or share photos on Windows Phone, you do so through a single interface, the Pictures hubs. All of the photos you have access to—including your own photos on the device, on online services such as Windows Live, Facebook, and so on, as well as the photos your friends, family members, and other contacts are sharing—are available right there, in a single place. But on an iPhone or other smartphone, there are numerous applications that provide access to photos. Some are in the Photos app. Some are in Facebook. Some are in Flickr. Some are Mobile Me, and so on. To access them, you must navigate in and out of different apps, repeatedly. Monotonously.

But it's not just pictures, of course. With Windows Phone 7.5, the People hub, through which you manage your contacts, now intuitively opens different communication channels on a per-contact basis, and depending on which services that contact uses. So with some contacts, you can use the usual phone, text/SMS, and email communication types. But others will have options for writing on their Facebook wall, chatting via Windows Live Messenger or Facebook Chat, and so on. Again, none of this requires separate apps that you have to know about, have downloaded and installed, and must find on some grid of home screens.

"This is the broader philosophy around Windows Phone," Sullivan told me. "A smartphone should be smart. It shouldn't rely on you; you should rely on it. Windows Phone 7.5 doubles down on that."

Apps, you say. It's all about apps. Well, Windows Phone actually has an interesting story there too.  First, although it's true that iPhone and Android have far more apps than does Windows Phone, recent studies of the "must-have" apps on both platforms shows that over 90 percent of them are already available on Windows Phone too, despite the relatively short time it’s been in the market. And there are in fact over 30,000 apps available for Windows Phone already, which is nothing to sneeze at.

But even the apps themselves are more sophisticated on Windows Phone than they are on iPhone or Android because of platform innovations that are coming with the 7.5 release. On iPhone and Android, apps are basically standalone environments, unaware of and unable to communicate with each other. With Windows Phone 7.5, apps can register themselves with the system via a new API called App Connect that allows apps to implicitly work with each other and with system services.

"If you're using Bing Vision [on Windows Phone 7.5] to identify a book and get more information, the Amazon Kindle app has registered itself as a solution for books and you can use that app to purchase the book immediately and wirelessly deliver it to your device," Sullivan said. "Other smartphone platforms have apps that can identify objects, and they have Kindle apps. But they don't offer a way for these two separately delivered solutions to work together. Only Windows Phone can connect them."

Windows Phone also delivers a compelling answer to the need of pervasive Internet access. Yes, it has the requisite standards-compliant, HTML5-based web browser with hardware acceleration—and yes, that browser is just about as good as the iPhone's Safari or Android's web browser. But increasingly, people are turning to web-connected apps, and in Windows Phone's more integrated user experience, this translates into a UI that’s infused with Internet connectivity. By logging on with your Windows Live ID, for example, you can sync SkyDrive-based documents to the Office hub and SkyDrive-based photos to the Pictures hub. The Bing app doesn't just return lists of web links but rather rich search experiences, including excellent local directories (the unique Local Scout feature), music search, Bing Vision (the previously described visual search service), voice search, and a much improved Maps experience that includes auto-rotating, as-you-go directions with voice, for free.

There are some previously unannounced features, too. Windows Phone 7.5 will indeed support tethering, or what Microsoft calls Internet Sharing, where you can share your cellular data connection with up to five devices, optionally with WPA2 wireless security. (This feature requires wireless carrier support and will be accompanied by additional monthly charges.) The new release supports hidden Wi-Fi networks, a curious omission in v1, though that will only be coming to new Windows Phone 7.5–based handsets at first. And Microsoft is enabling wireless carriers to add Visual Voicemail to Windows Phone handsets. Since AT&T provides this for iPhone users, I expect to see that on AT&T at the least.

In addition, don't forget that a web-based version of the Windows Phone Marketplace will be available by the time you read this, bolstering the on-device Marketplace and the version found in the Zune PC software. This newly updated Marketplace can wirelessly transmit purchased and downloaded apps to your phone, and because Microsoft is now tracking all your purchases and downloads, you won't have to manually manage that anymore.

Perhaps the best news about Windows Phone 7.5 is that Microsoft has learned from the issues it faced earlier this year when it tried to roll out smaller updates to customers. Although no one is making any explicit promises, the feeling is that this is going to go a lot smoother—and a lot more quickly—with Windows Phone 7.5. In fact, the first updates will start shipping to customer phones Tuesday at 10 a.m. PDT, and although the release will be metered at first, it will speed up over time.

"We have a pretty difficult problem to solve with updating," Sullivan admitted. "Our model is more like Android, a horizontal approach with different hardware devices, different wireless carriers, and different languages, and each one requires a unique image. Managing the process for updating is hard, but at least we're doing it. With Android, Google doesn't even try to update all the devices, not at the same time, and sometimes not ever." Apple, by contrast, has a much simpler time updating its iPhone devices because there is zero hardware variability.

"What we do is provide the best of both worlds," Sullivan continued. "You get a choice of the hardware and the networks, but we also bring everyone along when we issue a software update. We make it available to everyone and we try to do it as quickly as possible. We are going to do better this time. But we'll let the results speak for themselves."

I'll be posting my complete review of Windows Phone 7.5 over the next several days, starting today, on the SuperSite for Windows.