I think a lot of people seem to confuse "popular" with "overhyped". The game was marketed more than most games were, sure, but as far as I'm aware, Bungie didn't break any promises with the game. Whether you liked the Campaign mode or not is up to you, but as far as I can see, Bungie delivered the goods. Now Halo 3 is not my favourite game on 360. It probably comes in around 6th or 7th, but going online, all one sees is bitching about it.
A thread was made on this forum asking what gaming series everyone would delete from gaming history. I wasn't actually surprised to see that Halo came out on top, because most people claimed that it had "destroyed the fps genre" and bred a wave of unoriginal Halo clones. However, if one gooes back to before Halo, you'll find that there was nothing particualrly special about those games. You had Unreal and Quake, two series that highly resemble each other anyway, Doom, and Goldeneye (plus many more, but these four are the main players) All I can see Halo is done is add new features (or perfect/refine) existing ones. Mellee attacks, vehicular combat and grenades worked better in Halo than any game I had played to date, and whilst its not my favourite game ever, I would say that a great disservice would have been done to games in general.
And if the game happens to break all sales records when it is released, then good for Microsoft, who cares? Only Sony.
(For a more relevant post to the topic of "Should there be a Halo 4, see post 17)