A Windows Live ID is as integral to WP7 as a Google account is to Android, since it's your key to the app Marketplace and data syncing on the phone - in fact, it pretty much won't run without it. And some of the nerdier details: It's a 32-bit OS, with a kernel layer (the kernel, file system and graphics rendering) and a user layer (the graphical shell and apps). 3D graphics, BTW, run using standard Direct3D 11, not a gimped mobile version of DirectX. Apps get up to 1GB of virtual memory, with 2GB total for processes.
Oh, and apparently all system updates come via Microsoft's setup, which is nice, meaning its centralised - smaller updates over the air, and massive updates via Zune syncing software. There's lot more here, if you're interested in a deeper dive: [Tweakers via BGR]