Well first the enemy's. The Prometheans just look pretty stupid in my opinion, like some cross between Lego Bionic and this guy. Not very original and interesting to look at at all. Didn't they also say they removed the english VA from covie forces? I always love the little grunts screaming cool shit at me. Did they lose the ability to speak english? Is the translator node in Master Chief's suit broken?
The new weapons don't look cool or original at all. Sure they have space between them and aren't pieced together like human or covie weapons but still they look very bland.
Some other minor stuff like the crosshair looking way off when you shoot it and almost no recoil on the DMR or BR ( whatever was used in the E3 demo ).
Then you have the multi.. omg let's not begin there. Random weapon spawns, getting bonusses when you are on a hot streak ( like powering up your weapon after a few kills which gives you more damage < WTF ) Promethean vision etc.... Too many things to harp on but these seem pretty important to me.