Halo's eroding popularity can be pinpointed to six things. The AR, slowed down gameplay, overpowerd vehicles, bad maps/forge, bad playlist management, and armor abilites/equipment. Reach is the nail in the coffin but H3 is what poisoned the brand.
I thought I'd chime in on this.
1. The AR is not killing Halo. It's been in every game as a starting weapon. CE sort of not included since AR starts included the pistol. In Halo 2 we dealt with SMG's and some bullshit dual wielding combos. In Halo 3 we had a staple gun that was used for AR Rushing. In reach we have the AR which is better than its H3 counterpart in terms of damage and range, or at least it feels that way, but there's no bleed through, so it doesn't seem as bullshit as it was in H3. (Shoot 3 AR bullets + beat down anyone?)
I hate AR starts as much as the next guy, but at least we got the Reach pistol, nerfed as it was, to help deal with it. The scrubs need their AR's. And those who don't fall in the scrub category need their AR's because they can't appreciate the intricacy, skill and simple, balanced beauty of precision weapon starts. They need their AR starts to justify the "Weapon Variety" and turn Halo into a Rock Paper Scissors game. BUT AT LEAST ITS NOT BORING OLOLOL
2. I'll give you this. The AA's and thus, the movement speed, inertia, and jump height ruined Reach. Even messing with custom settings, the movement still feels off due to the heaviness of the inertia compared to other games. Players like feeling light on their feet. Its why things like Lightweight and Marathon are such popular perks in CoD. Why everyone wants to be the fast class. They love that agility. Bring that back.
3. *Overpowered Banshee. The rest of the vehicles are fine. Maybe the tank and gauss hog, but the gauss hog isn't on any map and the tank is a tank. I know you HATE vehicles in Halo, but you have to accept they are a part of the game and are more than just transports. Halo 3 had the strongest warthog, but aside from being stuck with a retarded team on Standoff, they weren't that bad.
I'd say bring vehicles to somewhere in between Halo 2 and 3. They should be treated as power items, used to break set ups and push into bases. I know you might disagree, but I <3 BTB and its infantry, vehicle interplay, and Reach ruined it by making the Banshee OP, while allowing the DMR and Sniper to become anti-vehicle weapons.
4. Yup. The maps were pretty bad. 343 needs to test the Halo 4 maps on EVERY GAMETYPE THEY WILL SUPPORT IN MATCHMAKING. If a certain gametype makes the map unbalanced, then don't have that gametype with that map in matchmaking. Note the initial spawns too, some teams have a huge advantage over others ie. Red Team getting rockets on Boardwalk, Elbow Spawning team getting guaranteed OS and a equal shot at Camo on Guardian.
90% of community maps have been terrible. The gray doesn't help, but its not just that. However, I think community maps are a good thing. But perhaps they should stay in a community playlist, and then if some maps are pulling ahead by being the dominant maps, put them into the regular hoppers.
5. Playlists seem to be fine now, despite Anni sort of making a mess of things. It's more of the things included in the playlist, ie. Drop Shields, Evade, AR starts in objective. The playlists have ALWAYS been shit at launch too. Hope they fix that. It seems to take FOREVER to get changes put into place.
6. I'm not opposed to putting in something new into Halo games, but the one rule it should have is to not break any of Halo's golden rules. Among them being that EVERYONE starts out the same. No Loadouts. The other problem AA's AND equipment had was visual indicators. I'd like to know if someone has camo, Armor Lock, sprint etc. But I'm not sure how to do this without resorting to silly auras around players or drastically changing their armor.
Anyway, equipement only suffered from that visual indicator problem. The AA's are all terrible though. They ruin gameplay and maps.