I was sad to see WebOS die because of the way it handled people like they were people, instead of accounts on services. I'm glad Windows Phone picked up that torch, even though I wish WebOS had succeeded. The only time I go into the Facebook app on my phone is to see who "liked" my status (no idea why the baked-in app doesn't show this) if it's a rare moment when I really feel the need to know this information.

Seamless conversations between text and IM is another feature picked up from WebOS that I feel is invaluable once you start using it.