Okay, since there are a few people uner the delusion that Halo had a good campaign, I'm going to tell you all the things that sucked about it. The gameplay was okay, and I did finish it more than once (sometimes I'm in the mood for mindless alien fragging), so I do have some idea of what I'm evaluating. I haven't played Halo 2, so I'm only commenting on the original.
1. Zero Bosses in the Entire Game: Not a single one. There are some tough enemies, and tough rooms, but not a single battle which felt epic. The only thing close to a boss fight is the escape at the end. This makes the gameplay feel very tedious, when all you're doing is killing the same trash you've killed a hundred times before. You just fight a little more cautiously when they use trash that's a little harder. You don't need to learn any specialized tactics after you learn when best to throw grenades and duck behind cover.
2. Story, or Lack Thereof: There are about four characters in the entire game, unless you count Foehammer who doesn't have a face. Of those four, only 343 Guilty Spark has any history whatsoever. Only the non-humans have any personality (ironic, isn't it?), and there is no character development in anybody. There's a skeleton of a plotline, which is mostly just used to describe mission objectives.
3. Linear Gameplay: Like most modern shooters, in order to ensure that the player doesn't have to worry about anything other than aiming his next shot, Halo is an extremely linear game. Personally, I don't enjoy being herded like a sheep, and this game feels like one big corral. There's one mission when you first land on Halo that lets you pick which of three objectives you go to first, and there are a couple times when you have to retrace your steps, but the latter feels more like lazy level design, which brings me to my next point.
4. Terrible Level Design: You're on a Covenant battle cruiser. Will the room behind the door be a big hanger, or just more corridors with dead doors? How exciting! Or maybe you're in the featureless guts of Halo itself, killing dozens of flood until you lose track of which way is forward and which way is back. Not all the levels are this bad, but the bad ones are soooooo looooooong.
Conclusion: Don't delude youself into thinking Halo's contribution to gaming was its campaign. Halo's contribution was very good (for the time) FPS controls on a console, coupled with excellent multiplayer. The alien-blasting gameplay was solid, and cool vehicles broke up some of the monotony of killing another room full of flood or another twitchy Covenant Elite. If the game had been single-player campaign only, it probably would have been worth 8/10.
Edit: One more point which didn't occur to me before.
5. No upgrade path: Some might consider this a feature, rather than a bug, but Halo has essentially no upgrade path. There are two powerups you can get and the shotgun and rocket launcher are powerful weapons which aren't available early in the game, but there's no long term upgrade system for weapons or durability or mobility. The Master Chief who lands on Halo is the same Master Chief who flys off it. Most shooters which preceeded it at least gave the player a broader, if improbable, array of carried weapons to use as the game progressed. Halo's 2 weapon system really did streamline the multiplayer aspect, though.
