I'll tell you exactly how I saw it.... (no story spoilers)
-character exiting a moving car at 30-40 mph, rolling one time, and then taking 5 full seconds to stand up
-simplified walking speed bumping mechanics (ie brick wall peds)
-emptier streets. esp when driving at high speeds
-invisible driver in the wall crash scene
-far less vehicle damage
-seemingly faster frame rates (game appeared to run very smoothly)
-shooting looked great
-minor vehicle damage allows more freedom to drive "balls out", and less boring trips to repair your custom rides.
-tennis looked snappy, challenging, and responsive
-In GTA IV you started out max level driving skill.
In V you improve over time. Once your driving is maxed out, it'll probably be a lot less slippery, and a lot more fun
-In a few of the on-foot scenes it looked like there were a fair amount of peds and cars
I am less hyped. But I still think the game will be a blast.