My point is coming from H1 and H2's smooth aiming and great headshot weapons felt like a regression. The BR in Halo 3 wasn't the accurate hit scan BR from Halo 2 or the Pistol from Halo, not to mention the horrible spread. On top of that Meele battles often ended in two deaths, something that rarely ever happened in the previous games. On top of that, the movement in Halo 3 was much slower than previous games; strafing was not as useful. In Halo 2 you had to shot the enemy and strafe to dodge his shots, in Halo 3 the footwork aspect was gone.
If you are a BTB fan I can see why you would enjoy Halo 3. The large sandbox and large maps lead to some really fun moments, however, BTB is only a fraction of the game. Halo 2 had some great BTB maps and moments as well but being on older hardware, they were not to the same scale as Halo 3. Halo 3 evolved BTB by introducing larger maps and new sandbox elements (elephants, choppers, mongooses, equipment) but the actual base game regressed.
I say all of that because I seem to remember you being a BTB fan and I believe that is probably why you like Halo 3 so much. I'm sure you would like Halo 3's large maps and expanded sandbox with Halo 2's basic gameplay elements (Hit scan no spread BR, faster movement and strafe, no double melee, etc).
My point being while Halo 3 was a great game, it was not the best Halo game when it comes to core mechanics. Halo 3's expanded sandbox was great and while the map selection was not collectively as good as Halo 2's selection, it did have some great maps. But for all of the improvements and new features Halo 3 brought, the core mechanics of the game somehow regressed.