-- The AMOLED screen has really deep blacks and vivid colours (and incidentally looks just as good as Apple's new 'retina' display on iPhone 4).
-- It's very responsive and never seems to slow down.
-- HTC's Sense interface is very customisable with its 7 screens and various widgets etc. I especially like the way it links your contacts to your facebook friends and updates their pictures and status inside the phonebook.
-- Android market has almost endless apps, and they're increasing in number every day.
-- Battery life. Mine lasts about 36 hours with moderate use, but that's pretty standard for a smartphone. Until battery technology improves, we won't get much more than that!
-- The camera's not great, but fine for snaps. If you want to take decent pictures, buy a CAMERA!
All in all, I can't fault it. Certainly as good as an iPhone 4, cheaper, more customisable, and you're not locked into using iTunes/iTunes Store.