The single feature I'm most excited about is the fact that it's supported -- I've been running open source ports of Android on my Sprint Touch (HTC Vogue) since before Sprint ever had any Android phones, and I'm ready for a real Android phone.
I also drool for its big screen, combined with the soon-to-be-released Kindle app -- no need to lug around a separate eReader when you've got an EVO 4G!