Obviously the gameplay. I could care less about graphics and couldn't imagine myself buying a game just for them. Plus, the gameplay in this one is truly awesome with the new (and returning) vehicles, large amount of side missions and good story missions.
Both gameplay and graphics make me want to play VCS, so where's that option? I don't understand how can anyone could "care less about graphics." If VCS had poor graphics but still has all the features it has now, you would probably view VCS more negatively (see GTA Chinatown Wars).
The graphics are pretty good, but gameplay is always more important to me. Obviously if the game had graphics like GTA 3, it would be crap, but they seem to be in-line with SA. If only SA had the new suspension system that VCS has, then SA would be 100%. Shame R* didn't bring out San Andreas Stories. That spoilt the collection.