4G data speeds
In smartphones, faster connections are better, and the Lumia 900 far outpaces the iPhone. It connects via AT&T's LTE service, and various reviews have clocked it at speeds generally ranging from 8Mbps to 20 Mbps. The iPhone doesn't have a 4G connection. If you're surfing the Web, streaming video, downloading music, and more, a high-speed connection like LTE makes a big difference.
The Windows Phone interface might not be superior to iOS, but in one area it has iOS beat: live tiles. The live tiles on the Lumia 900 deliver constantly changing information such as social networking updates without you having to do anything -- updates and data are automatically pumped in and displayed on tiles. The iPhone doesn't do that.
The Lumia 900 has a big, bright 4.3-inch screen. The iPhone's screen 3.7-inch screen looks puny next to it. Larger screens make a big difference when using maps, navigation, and browsing the Web. Admittedly, the iPhone's screen has a higher resolution, though.
At $100 with a two-year contract, the Lumia 900 is a steal. At either $200 (for the 16 GB version), $300 (for the 32 GB version), or $400 (for the 64 GB version) with a two year contract, the iPhone 4S costs between double and four times the price of a Lumia 900.
Better integration with Microsoft products
If you use Microsoft Office or a variety of other Microsoft services and products, you'll find the Lumia 900 far superior. It's got a mobile version of Microsoft Office built in, including the excellent OneNote mobile app, and Office integrates with Microsoft's SkyDrive cloud-based storage servcie. If you're not a fan of Microsoft products, this isn't a bonus, but if you do use them, the Lumia 900 has the iPhone beat.
By the way, I'm not saying that the Lumia 900 is superior to the iPhone at this point, just that there are multiple ways it's a better phone. For a more complete opinion, check out my Computerworld review when it's live.