I think Gothamite is suggesting the fact that San Andreas had stat building, which is a staple of the RPG genre (And, let's face it, RPG only means "Role-Playing Game" when it comes to tabletop games now. Video Game RPGs deal with grandoise stories, item management, stat building, and lengthy gameplay.), that it should have been an RPG.
I disagree, though. Otherwise you'd have games like Call of Duty 4 being considered RPGs.
I know, I was being sarcastic.
But in fairness, the general traits of RPGs have diffused themselves across all genres that it is actually pretty valid to call GTA an RPG in the sense that like many good games it has absorbed many of the traits of another genre --- in this case, RPGs --- in order to enrich itself.
GTA is not an RPG per se, but it does give you a large sandbox to express yourself as a player such that there's a big component of it that is RPG-like. In GTA you could just as well be a helpful ambulance-driving fella if the sociopathic ruffian path does not appeal to you or the main story track of an ambitious criminal climbing up the ladder is not your bag.
Some strategy games do the same thing --- incorporating statistically-based experience-reliant character development into 'hero' units or forcing the player to make moral decisions between what is practical to a mission over what is ethical.
A reverse example would be
Diablo II, which supplants a lot of the game world expression with character expression. Instead of giving players a means to affect the world ---
do you choose to raze the village for some phat lewt or do you save it and hope for deeper rewards from gratitude or unique prizes given only to the moral? --- the story-world is pretty linear and is pretty static in responding to the player.
Instead, Diablo II relies heavily on unique items and jewels and a very sophisticated ability tree and the game avatar is your sole expression of who you are... and it wraps this around an addictive strategy-game like playing experience with the a fast-paced rewards system --- the next new monster, new item, new gem, new level, new skill, a new thing every twenty minutes --- that resembles the tempo of an action game.
I haven't really thought this out clearly, but basically the best of games nowadays are trying to reach beyond their own genres in order to deepen and enrichen the play experience or at least give a fundamental twist that defines them as different from others. Hell, in terms of RTSs, nobody remembers pretty good games like
KKND or
Dark Reign because there was nothing that set them apart like the interlocking campaigns and factions of
Starcraft or the muscle-like momentum of
Command and Conquer's gameplay.