I came to WP7 from webOS, with only a very brief fling with Android in between. WP7, by far, is more related to webOS than Android is. It's smooth, fluid, and dead simple. Just like webOS. Apps just work. Games just work. Just like in webOS and totally unlike in Android.
I got an old 1st-gen WP7 phone and it's nearly as smooth and fast as the 2nd-gen phones, like the Lumia 900. However, with the 900 you're getting a *much* better value. There is no true multitasking like you're used to in webOS. But WP7 does fauxtasking quite well, honestly. The Zune music/video/podcast player is great. The web browser is far more capable than the one in any version of webOS. App selection is better.
You will miss the glowing notification LED. I sure do. And you may/may not miss the detailed volume controls (one volume for calls, one for music, one for ringer, one for alarm). Sadly, in WP7 there are just two volume settings: one for calls, and one for *everything* else. Yeah, kinda sucks.
You are tethered to the Windows program Zune to transfer pictures, music, videos, podcasts, and to update your phone. It's annoying, but workable. WP7 on any device is faster than webOS on any device but possibly the Pre 3 and the TouchPad. I have both.
If you were a big time Preware user, you need to prepare yourself for the fact there is nothing like Preware for WP7. XDA does a good job with various ROMs and the homebrew app market, but there's nothing at all like patches that we're used to in webOS.
I've been a WP7 user since February and I can tell you it's my new home. I really like it here in the Land of Tiles.