Microsoft's Windows Phone 7 made its debut in the U.S. today. We got a chance to give the company's new mobile operating system a spin over the last few days. Even though we have a few minor issues with it, Windows Phone 7 (WP7) is a welcome and much-needed reboot of Microsoft's mobile platform. The new mobile OS introduces quite a few innovative design ideas and should give the company a good shot at once again becoming a major player in the mobile business. Here are some of the things we enjoyed while using the phone, as well as some of the negatives we discovered during the time we spent with it.

Note: this is not a hardware review, but just for reference, the phone we used for this test was an HTC Surround. The Surround is somewhat of an odd device, as it looks as if it has a slide-out keyboard but actually features an extra set of speakers instead of a keyboard, as well as a "fold out kickstand for hands-free viewing and hands-in-the-air fun."

Good: The Metro UI Design

Few people think of great design when they think of Microsoft. With WP7's Metro UI, however, the company has managed to reimagine what a mobile user interface should look like. Developers have the ability to bring a fresh new design to all of their creations that is consistent across virtually all of the apps on the phone. As Paul Thurrott noted in his in-depth review of WP7, "one of the neatest things about Metro is that it gets out of the way. With Metro, your content - i.e. what's important to you - becomes the UI."

The design's focus on whitespace, modern typography and glanceable information makes it a welcome change from iOS and Android.

Good: "Glanceable" Homescreen

The first thing you see when you turn your WP7 phone on is a homescreen with numerous "live" tiles. This is a very different concept from what Apple and Google are doing with their mobile operating systems. The screen looks deceptively simple, with flat, monochromatic tiles, but these squares represent your email inboxes, phone calls, the people hub (which shows pictures of contacts that recently updated their social networking profiles) and other apps, all of which can update in real time. One of the best examples for this is the calendar tile, which always shows your next appointment. You can pin any app to this screen, which scrolls vertically.

A swipe to the right from there brings up an alphabetical list of all the installed apps. Microsoft decided to go for simplicity here, so there are no folders or multiple screens that allow you to organize your apps in different groups.

This homescreen is part of what sets WP7 apart from the competition and - in this author's opinion - represents a step forward from the experience on Android and iOS devices. On the other hand, though, once you install a large number of apps, WP7's focus on simplicity breaks down and folders would be a nice feature to have.

The unlock screen, too, includes a lot of glanceable information. Just by hitting the power button, you get to see your next appointment, as well as the number of unread email and unheard voicemails.

Good: The "Right" Apps are Available at Launch

There are already over 1,000 apps in the Marketplace and the U.S. launch will surely herald the beginning of a new wave of apps that will appear over the next week or so. Microsoft clearly understood that getting developers on board early and making app development easy was going to be a major factor in the platform's success.

The main apps you would expect to see on a mobile phone, including Facebook, Twitter, FourSquare, Netflix, the Weather Channel and others are already available. So are streaming music apps from Pandora and Slacker and social networking apps like Seesmic. There are also plenty of good news apps (including AP and Huffington Post apps), photo apps, e-book apps (Kindle) and productivity apps, ranging from PDF readers to a number of to-do lists.

By working directly with the developers, Microsoft also ensured that popular games like Flight Control, Guitar Hero 5, Need for Speed and the Sims are available on the phone, as well as some good games from Microsoft's own Game Studios, including the excellent The Harvest.

In the briefings we had with Microsoft in the last few months, the company always stressed that it was more interested in getting the "right" apps on the phone and not 200,000 other apps that nobody would be interested in.

Looking at the current lineup of apps, there are still some gaps in the lineup, especially when it comes to turn-by-turn navigation apps (though there are plenty of interesting mapping apps for WP7 out there already). Of course, not everybody has jumped on the WP7 bandwagon yet, so it's to be expected that some apps are still missing. Frequent travelers, for example, won't find any airline apps, and there is also no Instapaper app for WP7 yet.

Good: Hubs and Integration with Third-Party Services

WP7 makes extensive use of hubs. These are built-in apps that can aggregate data from a multitude of services (typically configured online on Live.com). The photo hub, for example, can aggregate images from Windows Live, as well as from Facebook, Flickr, SmugMug and others. The people hub works similarly and can display recent social media updates from your contacts. Third-party apps will be able to extend the functionality of these hubs, though we haven't quite seen this in action yet.

Microsoft didn't shy away from integrating third-party services here. This may come as a bit of a surprise to some, but this move is quite consistent with how the Windows Live team, for example, currently approaches the online ecosystem.

Mixed Bag: Giving Carriers Their Own App Stores and Ability to Delay Updates

Carriers and OEMs can run their own WP7 app stores and our demo unit featured both an AT&T and HTC store. Carriers can also pre-install their own apps on the phone's homescreen. While this is somewhat annoying, the good news is that the carriers can't stop you from deleting these apps.

In another concession to the carriers, Microsoft is also allowing them to stay one step behind Microsoft's official release cycle for WP7 and delay updates. Given that Microsoft is setting rather stringent rules for the minimum hardware in every WP7 device, this is somewhat odd. On the other hand, though, this is still better than the situation many Android users find themselves in, where they can never be sure when and if they will get the next version of their operating system.

WP7's Achilles Heel: Internet Explorer

The browser is one of the central apps of every modern smartphone and sadly, Internet Explorer can sometimes drag down the overall positive experience with the phone. We ran into numerous issues with sites that just didn't work or look right. Among these were Techmeme and Digg's mobile site. We didn't come across these problems too often, but when we did, these were usually showstoppers and made for a rather frustrating browsing experience.

For the most part, Internet Explorer on WP7 felt more than fast enough for browsing most sites, but the browser sometimes slowed down to a crawl when looking at more complex AJAX-driven sites.

There are, of course, still a number of other minor issues and missing features. For the time being, for example, there is no way to copy and paste content. There is also no unified inbox for those who need to juggle multiple email accounts, and the lack of folders makes managing large app collections harder than it should be.

That said, though, Windows Phone 7 is fun and easy to use. It doesn't just bring numerous interesting design ideas to Microsoft's mobile platform but introduces quite a few novel concepts that its competitors will hopefully bring to their own platforms as well.