I think this game nails the atmosphere in the exact same way that RDR did. I did a complete circuit around the map last night, and wound up driving back into town from the desert as the sun was setting. Just the weather effects combined with the atmospherics and colors were spot on perfect. And dat music
Driving in the rain, the water effects on your tires

In a lot of ways it reminds me of modern day Red Dead more than a follow-up to GTA4 (as dumb as that sounds).
some random bits I haven't seen others mention:
-The seamless transition from gameplay to cutscene back to gameplay is great. You're used to arriving at a mission market, fade to black, cutscene, fade to black, back to gameplay. In GTAV you sometimes don't notice that cutscene has ended and you're in control again (that a good thing).
-RPG abilities, uh, you can max out your gun skill in ~5 minutes at the gun range. The other abilities aren't as quick, but it doesn't come off like you'll need to grind or anything.
-Random crimes and events happen like in RDR, and so far it's more varied than in RDR. I've stopped mugging, smacked around paparazzi, saw a gang vs police shootout, etc. All in all it makes the world feel active.
-Way too much of the writing is soap box preaching.
Way too much. That said, the characters and stuff are better than GTA4 already.
-Retractable roofs
