Oh boy, where do I start?

Campaign:

Bad enemy design
Bad level design
Bad writing
Invisible walls

Multi:

Custom loadouts
Ordnance
Armor abilities
No red Xs
Bad maps
Dumb new medals (single kill? Really?)
Horrible gametype changes
Imbalanced
Several custom gametype options were either missing or didn't work

Both:

Worst UI ever. Of all time.
Sprint
Slight bloom (on DMR)
Flinch (as opposed to descope)
Shitty online stats

Cut features:

Playing as an Elite
Campaign theater
Zoom/percision control in forge
Firefight
Rendered films
No screenshots on their website (at launch)
No in-game file search (at launch)
Several popular gametypes were cut
Several emblems were cut

There's probably a lot more I'm blocking from memory.