I know a lot of people are complaining about this. I just want to make my own list.
Forge:
-“Fine tune” feature from Reach. Why did you even remove this? I see no reason for it. Magnets do not replace it.
-Zoom feature in monitor mode. Again, why did you remove this?
-More objects that are effected by gravity. Without this, no Jenga tower or trash compactor 
Custom Games:
-Zombie/Flood load out customization. By removing this you pretty much deleted all of the most played custom games from Reach and Halo 3. Fat kid, Trash Compactor, Halo on Halo etc.
-Disabling sprint. So that we can play classic Halo if we want to.
-Where did VIP, Juggernaut, Assault, Race and Invasion go? You won’t exactly make your fan base happy by removing game types that there was nothing wrong with. I don’t get why you remove things? Why not keep game modes and add new ones on top of those?
Assault, Juggernaut and Race were classic Halo game types, why take them from us?
-Changing color of humans/zombies. Just a minor thing I think is missing from the options menu.
My playlist wishes:
-Rumble pit. This one has been around since Halo 2, why remove it?
-Assault.
-Invasion. Loved by many in Reach.
-Multi Team.
-Double Team.
-Head Hunter. PLEASE?!? I thought this was fun as hell! 
-Stockpile. Awesome and different objective game mode.
Thanks for adding SWAT and Team Snipers, hope to see more playlists returning soon!
Please tell me if I missed something, I’ll add it to the list! What are your thoughts? 