I am spending about an hour every day playing Halo 2 on my PC while stepping on the treadmill, while Xbox 360 is sitting on the side, collecting dust.
(1) Matchmaking on Xbox live is terrible! This is not actually a feature of Halo 3 per se, it was the same towards the end with Halo 2. How do I know? Because most of the games I play end up with extremely unbalanced teams. I am willing to bet that if anyone at Microsoft were to care to look at the stats, they would have discovered that maybe only 10-20% of the games end with close scores (2:3, 4:5, 3:5, even), whereas >50% end with one of the teams scoring 0.
And when you get either beaten all the time, or win all the time, it's just not fun. This situation was actually becoming worse as time went, from the initial matchmaking when Halo 2 just shipped, which was not indecent, to what it is now.
One might also think that creating a matchmaking environment where teams are balanced, as evidenced by the scores, is a very decent, and measurable, review score. Perhaps not.
On the PC there is no automatic matchmaking. One joins whatever game he or she likes from the list. If there are more strong players on one team than the other, people tend to migrate to the other team until things balance out.
(2) Boy, does Halo 3 have weapons! When Halo 1 came out, it had 10 Spartan-usable weapons, including 2 types of grenades. Now in Halo 3 there are whopping 12 human and 14 covenant weapons, and that does not include the vehicles!
When you have a matrix of 26x26, it is impossible to talk about balance.
The power here is NOT in numbers!
(3) They changed the controller button mapping - and in a completely unreasonable way. There was one notable problem with the old Xbox button mapping - to punch, one needed to move the finger off the direction thumbstick, thus not being able to turn while trying to punch.
With the new set of buttons close to the triggers, it looked like a perfect opportunity to fix that and place the punch button where it actually can be accessed. Instead they randomly moved weapon loading functions to these buttons. Why?
(4) Halo 1 was extremely innovative game because instead of putting the player into an enfilade of rooms where you have to kill all the monster in Room 1 before getting to Room 2, it gave a player a relatively open physical space controlled by certain laws, but where there are unlimited number of solutions to every problem.
Halo 2 was very innovative because it went a very long way towards solving typical networking problems of a multiplayer game - it handled lag an order of magnitude better than anything before that, and the matchmaking algorithm was not that bad from the start.
Why would anyone call Halo 3 an innovative game? Because they did not broke (completely) anything that did not need fixing?
The article in Wired about Halo 3 usability hints that the development team was not particularly proud of Halo 2 release - they felt that the story was dropped on the floor at the very end. I am wondering if the Bungie team is proud of Halo 3. Judging from their recent separation from Microsoft - maybe not...