Coming from a Hero the last 2 years, and living with the Nexus since debut, I'm thinking:
Screen, I love it. The other phones' screens have lost their appeal to me now, looks a bit outdated.
The simplicity of design
No flashing notification lights (yes, I've come to appreciate that)
Clarity of voice calls
4 options on the phone menu..phone, call log, contacts, favorites
Voice dialing works great for me (Hero not very well so didn't use it)
Battery life
Navigation, works tremendously well for me (Garmin is now retired)
The knowledge that updates can be provided quicker
Google Voice - Only have had it with this phone, it's cool. Already gotten compliments from people calling that leave voice mail, they don't have to listen to the usual Sprint voice mail speel. And the fact that to get my voice messages I don't have to sit through the usual Sprint speel either. It's right there in text, and for voice push a button. Speaking texts is cool, as well as sending texts etc from computer.
Size and weight
I'm not a phone guru, I'm sure there are other phones with some of the same features, but this phone simply puts it all together in a slick package.