Personally, I dislike the fact that there are fewer gametypes and opitions in customs. Flood is also a disappointment due to the lack of opitions. Problems in Forge such as lack of persision movement/zoom. Theatrer only being available for War Games. Finally, Sparton Ops was just terrible, bring back Firefight!
We have Halo 4 Discussion and a bunch of other threads about bugs, nerfing and buffing certain mechanics, etc. already.