Grand Theft Auto: San Andreas
As the story proceeds out of the Los Santos area, the gang war system goes into hiding, giving way to more traditional run-around GTA missions. Problem is that this game world is huge, and missions have a depressing tendency to order you across the map and back again on a whim. If San Andreas were any other GTA, this would simply be annoying - but in SA, a lot of the ground you have to cover isn't cityscape, but fields and farmland. It's boring.
So this is why I've been making extremely slow progress in San Andreas. To a degree, I think this game may be too big for its own good. But given that I still haven't visited the game's Vegas-spoof city, I do intend to soldier on.
Progress: San Fierro