Oh, so much - in all honesty, this was one of the darn best games I ever played...
I really enjoyed talking to people and overhearing people talk even more than before - I found myself doing a complete tour of the Normandy and the Citadel after every completed mission, just so I wouldn't miss anything. And not only the squadmates. People all over the place pulling themselves together.
Like the Turian taking care of the girl who was waiting for her parents at the refugee camp.
The C-Sec officer planning to quit his job and go to Palaven to find his buddy's family.
The human woman and the salarian guy outside Purgatory, with his new shiny armor.
Also, Mordin. From finding out he was the one feeding Wrex information on the Krogan females, him taking care of "Eve" (Krogan Queen, anyone), the exchange while fleeing Kalros ("Kalros still chasing us!" - "Tell me something I don't know!" - "Metal in truck excellent iron supplement for thresher worm's diet!") up to his heroic sacrifice. Many a manly tear was shed.
And while I never cared much for Grunt in 2, his "sacrifice" scene was just beyond awesome, from "My turn... heh heh heh..." to "Got anything to eat?".
But it's hard to put my finger on it, and I already feel like I'm doing many who go unmentioned here a grave injustice.
In short: The characters, in writing and (voice-)acting. Everyone did one hell of a job there.
Except for Renegade Shepard. I hate that guy