UC3 MP was fucked up from the start:
-Situational Awareness was promised to be out, but they just ended up breaking it up into like 5 different things (boosters, power plays, paid boosters),
-they added a sprint which was not too bad, until they made it much more responsive, which I didnt like
-the maps were WAY too big. This was a problem since they started doing UC MP DLC. I swear, the classic maps are so damn good-- smaller, more symmetrical, etc.
-Kickbacks/scorestreaks. Not bad in itself, because they supported the game with a basic hardcore mode, then later a classic mode, but the kickbacks were bound to cause balance issues, and they did.
-class loadouts
Really, what made UC2 so good was that it took from Halo: simple maps, map control-gameplay, weapon pickups, everyone starts out virtually on the same playing field, boosters were the only differentiator.
They threw that all out in UC3, added all kinds of carrot-on-a-stick mechanics to desperately keep people coming back. Now it's free to play and they will milk the game with stat-changing attire, tons of taunts, and stuff like that.
It's not all bad, it's just such a shame how far it fell from UC2 (which was fucked the minute they patched health anyway)