I've been on WinMo for years and 7 leaves me cold. I was happily enough putting up with WinMo's horrible interface (which was less of a problem due to my phone's awesome 6-row physical keyboard), it's ridiculous slowness and its lack of customizability, because in exchange I was getting full multitasking, a decent mobile version of Office included free in the ROM, and a decent developer community (not as huge as the Android and iPhone dev communities, but I'm more interested in mobile ports of Nethack and SCUMMvm than in a new flashy Twitter app, which seems to be what the Android market is mostly populated with

).
7 takes away the multitasking, takes away the ability to run native code (so lots of developers have fled in disgust), and will probably be locked down tighter than an iPhone so you won't be able to install all the cool little homebrew apps that those people used to make. It feels like Microsoft is abandoning people like me and going after all the bleating, screaming, tweeting, facebooking teenagers, all because they're a cooler and hipper demographic.
Joke's on them though, cause they're basically trying to steal iOS's core audience, but iOS has moved on from being the paranoically locked-shut OS it started out as: they're allowing limited multitasking, and have opened up lots more APIs for developers to use that they previously only kept for internal Apple coders. WP7 is like iOS 2 versions ago, and I reckon they're gonna fall flat on their face when all the screaming teenagers fail to be wooed by it, and then they'll wish they hadn't abandoned their small but loyal demographic of more serious users. People like me will move to Android, RIM (and, eventually, Meego and WebOS 2.0 if Nokia and HP pull their thumbs out).