If you think Halo was running in a tunnel, you've only played one level. You need to play them all, from start to finish, and be attentive to everything, because unlike some games, Halo is actually more of an experience that has to be had as a whole, or it won't have nearly as much impact. Because everything in the game contributes to it. The art direction, sound direction, the story, the way the story unfolds, the environments it takes place in, and the hundreds of little clues that suggest tales of mysterious thingies around the galaxy. The best thing is that most people won't even notice those things even the second or third time through the campaign.
That's what makes the first game better than the sequel, also. Halo 2 just had a less interesting story... even if the gameplay, technically, is better.
And despite what you just now may or may not have read, I'm NOT a fanboy either. I'm just very interested.