Gotta say, I wasn't too much a fan of being killed in one hit by a baseball bat to the head in IV. As realistic as it was, it frustrated the hell out of me.
But, when I was lucky enough to do the same to an NPC, my feelings were far different.
EDIT: Also, maybe like in MGS3, we'll be prompted to choose what GTA we like best to have certain HUD options change...
I like GTA III! Numbers that reflect health and armor percentage are shown in the HUD.
I like GTA:SA! Colored bars that reflect health and armor percentage are shown in the HUD.
I like GTA IV! Health and armor are indicated by bars that wrap around the mini-map.