Thats why Microsoft never made a big deal of Halo going to pc - Bungie did, mostly because they got to throw in some new stuff. Halo games thrive on the multiplayer action of sitting next to your friend, blowing his head off, and 20 minutes later laughing at him about it as you get chips and soda. It's a console game at heart (at least now, it was originally going to be a game for Macs), and the game is designed for that.
Halo 2 was complicated? I agree, mostly because I loved Halo 1's simplicity, while still allowing cool things to be done of all sorts. I was never a huge fan of Halo 2, but supposedly the game play (coming from a friend who participated in the beta) is more like Halo 1 than Halo 2. Well, in an hour we shall see (some of us at least).
Where did you hear about the story mode being that short, by the way? I don't believe that yet, considering Halo 1's story mode was nearly 10 hours, Halo 2's about the same, and there are so many loose ends of plot to tie up....that would be insane if they could pull it off in a 5-6 hour game though.