I don't understand why everyone says the missions in GTA IV are so boring. I think they are the best in the series. People say they are too "samey" but I actually think that's a plus. The game's best mechanics are shooting and driving, so why not have those are the main mechanics for the missions? I would much rather have missions where you shoot guys than going in a submarine.
I feel like they also do a good job of having different, creative missions from a simple framework. Virtually every missions is driving or shooting, but they are just different enough to feel fresh. For instance, one mission you have to chase a train. Another, there is a scooter chase throughout Middle Park. The shooting missions tend to put you in interesting scenarios: a shootout in a construction site, or a museum. Or one where you're in a drug deal, but the dealers turn out to be cops, so you get into a shootout and have to escape. Simple and elegant. In comparison the missions in the other games often feel gimmicky and poorly designed; this is the only GTA where I can say the missions are actually the most fun part of the game.