No more map/weapon/power up control. No more long, fun vehicles battles since they're made of paper and practically every weapon can take them out. No more "**** it" playlists to mess around in as Rubine players. No more deathball in Forge. No more fun items like the soccer ball and golf hole.
Gone are the times when someone could, and would, make a complete optical illusion of a playable map in Forge with just the walls and blocks in Foundry. Gone are the Rube-Goldberg machines. Gone are the Jump Challenges. Gone are the "Saw-esque" mazes that involved things like grabbing turrets through walls. Gone are the breaks from MLG customs to play Jenga.
We've said goodbye to literal Halo classics, such as Assault and One-Flag. (includes Neutral Flag/Bomb) We've said goodbye the combination of Juggernaut and negative points for kills, and therefore said goodbye to customs like Tag. We've said goodbye to the Brute Shot and are forced to play dodgeball with that aborted contraption they call the Concussion Rifle. And unfortunately, we've said goodbye to customs like the real Grifball.
Remember when the Carbine was almost better than the BR? Remember when the only anti-vehicle weapons were the rocket launcher and Spartan Laser? Remember when the combat wasn't slow, but it wasn't so fast that people stopped thinking? Remember when your dead enemies always dropped grenades? Remember the mandatory 3 second respawn in Slayer? Remember BTB 2-flag on Avalanche? Remember Team Snipers on Headlong? Remember when matchmaking gave you actual choices instead of illusions of choice?
While I still think H4 is Halo - and more of a Halo game than that game ever was, I mostly agree with all your points (though I'd argue that H4 has the fastest paced combat since CE)
I don't really care about custom games personally, but the thing that drew me to Halo multiplayer from the very beginning in 2001 was how competitive it was, and ever since that game, the developers have decided that competitive play is not going to be the core of Halo any more.