aside from the typical things that people like me complain about i'd say my complaint is something that isn't confirmed or unconfirmed but i know what be there in the same capacity i want it to be. custom game creation, halo 1 pc had great content, especially because of mods, halo 2 had fun and variable customs, halo 3 was the pinnacle because of all the small glitches and physics oddities it allowed players to exploit that system to create very intricate custom game experiences, halo reach had some fun ones though it was less diverse and structured than the halo 3 ones and halo 4 was lacking.
to me halo is like a beautiful trinity where they merged something akin to mario sunshine, smash melee and mario party 4 into one game. a fun and replayable single player experience, competitive games, party / custom games and the ability to really modify and tailor your experience, something that i feel is not part of 343s design philosophy and what ultimately lead to the insane amount of playtime i racked up over 11 years.