1 - Highway driving like San Andreas, and lots of countryside.. Small towns too! GTA San andreas kicked GTA4/s ass in this regard. I loved driving the highway to the different areas in san andreas for the first time.
2- The city parts in GTA4 all look the same, a little more uniqueness in different areas so they stand out more - seems like the past GTAs did a better job of it. More uniqueness..
3 - More buildings to explore, more to do. Mall maybe, or a zoo, we just want more, not a stale barebones city that only has the illusion.
4 - Better storyline, a more likable character; the GTA3 quiet guy was perfect - heck bring him back. And perhaps mix it up let us play a couple different intersecting storylines/characters. I got tired of Niko fast, i doubt I'm alone in this.
5 - More Easter eggs/ side missions.. I liked the rpg elements of San andreas way better than the friend/GF 'chore' of GTA4. GF was ok in san andreas, as long as they imply you can visit them when you please - the cellphone kind of implied i had to detract from missions just to keep them liking you.
6 - Smoother frame-rate, better graphics. As good looking as it can be, GTA4 chugs. Less loading times on things like shopping. Look at something like Infamous and how smoothly that open world runs. More weather, I loved the thunderstorms in San Andreas.
7 - Checkpoint system like Episodes from lib city.
8- Co op, storyline of the main game, not side stuff. Split screen would be awesome, even if it's limited like the SanAndreas coop. I just don't see GTA doing multiplayer well, unless it was on like a MAG scale. Co op would work though.