I like the fact that they have added numerous things to it: you have to watch your weight, you can have 3 people hang out of your car and shoot while you drive, the area of San Andreas contains 4 cities and is 5 times the size of Vice City (which is twice the size of GTA3). There's always more vehicles, you can get haircuts, outfits, etc. If you eat too much (watch your weight above) your character will get fat and run slower, so if you work out in the gym more, your character will look like a body builder. Plus, it's just fun to wreak havoc on the city.
I still wish they were smart enough to have multiplayer though... Even if it was 2 people each on 4 consoles (8 total) that would be incredible. They'd still need to have all the civilians and cars in the game to make it fun, and it'd also be cool with game modes like capture the flag, but I don't want to ask TOO much...