Since you've already talked about this game I guess I don't have anything else to say... oh wait, this game is massive. Finding something else to say shouldn't be very hard.
Lets start with the soundtrack. The GTA3 series has some of the best music (not composed for that game) around, with San Andreas being a particularly high point. There are ten different music stations with music ranging from The Ohio Players to Public Enemy to Soundgarden to some techno or reggae band you'd likely know if you were into those genres. Each station is based on one you might've heard during the mid-to-early nineties so it's almost certain to be a trip down memory lane for anyone who plays. And of course, the djs only help to fill such a memory. The greatest of these for me was the host of Radio X, the alternative rock station. While completely satirizing the people who appear on such a station, she managed to say almost exactly the lines used by the host of local alternative station The X 103.9 while cutting to, in some cases, the same music (Mother by Danzig as an example).
One of the cool features added to San Andreas is the gang wars minigame. The object is to claim turf by entering a enemy controlled area and taking out a few waves of bad guys. The larger the area of your gang's influence, the bigger the monetary payoff you receive. Every so often your territory will be attacked and you have to chase off the enemy gangs to continue earning revenue.
You've talked about the open world already, Dave, but of course that's the biggest draw to this game. Anything from jumping out of airplanes to gambling to pissing off the cops and outrunning them and the freedom to explore the entire tri-city area is what makes this games so great.