I think part of the reason Halo endures for me is that it's the (console) FPS that seems most like a "sport". It's colourful and bright, the rules and scoring are solid, it's statistically deep, and things feel symmetrical, or at least balanced, in terms of how the teams square off. (And the Arena is going to take this to the next level; it really is going to feel like a virtual sport, and I love that.)
Matchmaking does its best to help this: it at least tries to create fair contests between determined teams of players, who are there from the start of the game and see it out to the end (provided they don't quit, which is the biggest issue with the model).
Years of this has put me completely off the drop-in, drop-out of MW2 or games like it. I can start a game in 10-15 seconds, sure, but I might only be catching the tail end of a blowout... or I'll start a game with a few people but over the course they drop out and others replace them, so I don't feel a sense of continuity. There's no individual "game" I can focus on, just a series of gamelike events with random people stumbling in and out (with guns).
In Halo, unless people quit, you're getting a match-up. One team (or player) wins, the others lose. It feels like stepping onto a playing court or field, and I'm happy to wait another minute or two if it improves the reliability of that experience.
Fixed in Reach. The lobby arrives without loading a map, everyone votes for the combination of map and gametype they want (there's also a "None of the Above" option) within a fixed timer, and only then does the game load the required map.