The two main reasons that I didn't like Halo 3's Campaign was because of Brutes as the primary enemy, and too many vehicle focused maps. I prefer the balance of Halo CE, where most of the levels are based on infantry combat, and the vehicle sections are either quick and focused or used sparingly. Only AoTCR and Halo are the levels that are primarily vehicle levels, and even those have a lot of great infantry encounters. In Halo 3, I never liked that I was fighting Brutes the whole game, and I have never loved Halo games because of the vehicles, so it just felt like too much of Halo 3 was made up of vehicle combat.
Halo 3 is my least played Halo Campaign, and I honestly can't even remember most of the second half of it, I ran through it on Legendary 4 player co op in like 4 hours. I never really returned to it because the gameplay of Halo 3 was lacking imo, and because of all the damn Brutes and vehicles. Every time I would try and give it a shot in Solo campaign, I would become bored in 30 mins or less and quit.