By the power of truth, I, while living, have conquered the universe.
Armour Abilities. As long as these are in Halo, Firefight just can't be good. In Reach, they're a core part of the gameplay - the golden triangle becoming a tin quadrilateral. Disabling these just makes the experience so unnatural, even the Classic Slayer gametypes in Matchmaking just felt wrong without AAs.
In ODST, the only equipment you get to rely on is the occasional Bubble Shield a Brute threw down before you killed him.
Also, maps. All of them are terrible in Reach, the exception being Installation 04. Crater, Lost Platoon, Chasm Ten, the Marathon-inspired Last Exit, Security Zone... pretty much every map except for Windward was great. Cruising around with Choppers on Heroic with 3 of your friends was fun because the game was so vehicle-friendly, the dumbing down of vehicular combat in Reach just didn't work.
Custom options ruined Firefight. Every game was a Jetpack-fest with unlimited lives and Rockets that had infinite ammo. Yeah, you could customise the rules to a more ODST-style type of gameplay, but please do tell me exactly how many private Firefight matches any of you played with friends... It just wasn't the same, Commendations also played a part in making FF worse because all people did was grind for more cR. ODST had the benefit of rewarding you nothing beyond an Achievement and a pretty skull on Bungie.net.