The multiplayer aspect is what made the franchise as big as it is. While the story itself is not bad, it usually goes overlooked by people who just want to sign into Live and murder n00bs.
As far as gameplay is concerned, I don't think Halo has brought anything new or exciting to the table. Its just a quick FPS. I honestly don't know why so many people get all ridiculous over it.
Goldeneye and Perfect Dark gave us customization and Timesplitters gave us map creation and bots with adjustable AI. Counter-Strike gave us tactical gameplay. Half-Life was great, and Half-Life 2 had that Gravity gun, which allowed us to make creative ways to complete tasks and kill guys with couches and saw blades.
As for Gears of War, there was almost no storyline, but the gameplay was great, which made it fun to play. I think the reason it isnt bigger than it is is because of how the multiplayer is played. The Gears multiplayer is slower because you have to hide more and pay attention to your surrondings, because if you don't, you die and have to sit out for the rest of the round. Halo players (in general) don't like playing like that. They would rather grab a gun and charge blindly into battle, die, respawn, and repeat. This is why they opt to go back to Halo 2, which is considerably faster.