Because Halo is the definition of the word Basic, there's a reason so many people play it, you pick up a gun, point at something in your reticle, and shoot. There is nothing that separates it from any other FPS other than the fact that it DOESN'T have anything unique.
I've played Deus Ex, which had an incredibly deep mechanic system which you could fine tune how your character plays.
I've played Tribes which featured different classes of players, jet packs, huge maps, and an awesome ability called skiing which basically let you slide against terrain without friction slowing you down, which let you gain momentum on hills and stuff. Really hard to explain but totally friggin awesome.
I've played Unreal Tournament, which had some of the coolest weapons I've ever seen in a game :P
I've played Quake which also had some intense matches, very fast paced and again some great weapons and mods.
I've played Battlefield and Codename Eagle which featured some of the greatest vehicular fights I've ever seen :P
Recently I've gotten major hardcore into Resistance fall of man, which the greatest weapons I've ever had in a video game, every weapon offers you a secondary fire option which gives you a MASSIVE amount of control and strategy on how a particular weapon could be used in a number of different ways.
Then we go over to Halo, which is hailed as god amongst games by frantic 12 year olds, and you can start to see why. The action is VERY basic, you can pick it up and know what to do. It's got a learning curve that a 4 year old can understand. It also happens to have some of the lamest weapons I've ever seen, the development team was incredible large yet they somehow managed to pick the most basic weapons in the world and make them sound like ass. Paired with the terrible audio is the hit response, or lack therefor of, when you shoot into an enemy and you have to plow through there shield, you really lose that feeling of power, your weapons literally feel like toys.
I hyped myself up so much for Halo 3 and I felt that it was one of the saddest experiences I've had in a long time. The story in the 3rd was horrible, the dialogue was un-interesting, and the ending suuuuuuucked. I just for the life of me can't understand why anyone praises this game, it offers NOTHING that I haven't seen before and better. There's a reason so many 12 year olds play it, because they can without needing to have previous skill in FPS's.