People always say San Andreas has tons of stuff for you to do, but the quality of it isn't that amazing. Unlike in the other games, there's a multitude of collectibles. Oysters, horseshoes, photos and gang tags. But they're all the same really. You have more races, more stadiums than Vice City, but they're just the same thing. Nothing unique to them. Yeah, the map is big - but it still feels pretty empty.
The high point for San Andreas is the customization and stats, people really seem to like that.
What's unique to San Andreas is the Gang Wars, but that's there because of the theme of the game. Just like how Vice City had businesses you had to buy and do missions for, that was the theme: rising up and having a huge front for your drug business.
Yeah, there's a whole lot to do in San Andreas. It presents itself as having lots of different activities, but it seems like an illusion to me.