Eep, wasn't expecting so many people with a similar 'slump' situation as me.
When I started playing the game I was enthralled - even on my cruddy x1900xt. Had some real fun being chased by the low-level cops, parkour-ing my way around parking lots and industrial areas before hotwiring a cadillac-wannabe and screaming my way into the (then locked) second island only to bring down the wrath of the police force. Ducked about various alleyways, barely dodged SWAT vans trying to ram me with all they got... Eventually found myself one of those oddly 'scary' looking parking lots made out of that big steel girder construction (hard to explain that), hid out there, but the cops eventually found me and I screamed out in another stolen vehicle to have probably a good 30 minute chase through the parklands and whatnot. It was GREAT.
Then it just... died. I think that once I got to know the gameworld that much better, I suddenly felt much less like I was in this sprawling metropolis with endless little places to explore and experience, and more like I was in this confined, limited little world constructed more or less for the purpose of conveying one single storyline and not much else. I mean, once you tear through X amount of streets and blow up X amount of cars or hotdog vendors or SWAT vans or whathaveyou, there's not much beyond it. I didn't (did not and still don't) get this feeling with SA, for whatever reason. Probably has to do with the diversity of the environments.
It's hard to explain... But even with how incredible they built the game world, even with how alive it feels, something is just missing. If it were all twice as big (SA size? Oh drool), I think that would make an amazing difference. As it is I do really really really like it, but all the various areas are quite samey. Maybe I'll see it differently when I get back into it again, whenever.