It kinda felt like an unfinished game to me. There were several good ideas in terms of gameplay, but they were overshadowed by everything else that was wrong.
What was wrong Regarding the multiplayer : You couldn't join a game that had already started.
Game modes weren't as flexible as in AvP2, and there wasn't as many.
AvP2's system of class was completely dropped
exit runners/predaliens/preatorians, heavy gunners/sniper/else, *insert predator classes names here*
there was also some balance issues of course
Regarding the singleplayer : They tried to re-attach themselves with story of the movies ...
The AI ... where should I start ?
the Alien AI seems decent at first, it's quite stealthy, sometimes it dodges stuff, but isn't quite aggressive enough, especially when there is a lot of aliens around and tended to use the heavy and slow attack, even when you weren't blocking
the Predator ... hard to tell since you only fight them twice in boss fights
the Marines ... were complete morons. It was like someone took a CoD AI and put it in the game unchanged. They wouldn't change their patrol pattern even after seeing half their buddies dead, or even think about looking up (among other things). They could have at least scripted them to regroup once they discovered that several of their teammates are dead
Else : can't crouch ...
there wasn't as many weapons as in the first 2
no rocket launcher, no grenade/mine launcher, no minigun (but them smartgun was more powerful to compensate)
no pistol, no net gun, no melee spear, no harpoon rifle
you can toss only one flare at a time
the Marine can punch a two meter tall armored killing machine/two meter tall hunting-killing machine ... even schwarzy didn't had much result when he tried that
the motion tracker only pings enemy movement, that means no "Is it an alien, an animal or just some piece of machinerie/else" moments
the turrets don't *beep* anymore. I
liked the beeping turrets

can't cancel a finishing move
aliens exploded when killed by a flame thrower + flame thrower slightly more powerful than in 1 (= uttercrap compared to the other weapons). For the record, the Flamethrower's qualities were 1) aliens don't spray acid everywhere when they die (AvP 1 & 2) and 2) is actually quite powerful (AvP 2)
I want to drive a #*!ø®ng APC since AvP 2!
What was good I liked the HUDs
the Alien is a lot more agile, it is a lot easier to move on the walls. You really feel like a killing machine.
likewise, the Predator's super jump is a lot more accurate, making you more agile.
modern graphics = more realistic gore & stuff, pretty pred-cloacking, pretty thermal vision
pretty finishing moves
the new smartgun is
awesome !
no more uber-powerful pulse rifle
new shoulder canon is more powerful and less dangerous for yourself (although ... see balance issues above)
gameplay is even more differente than before between the 3 species. It really feels like you are playing a completely different species
alien vision adapts itself to the ambient lighting.
aliens can see through walls (see balance issues above). It makes sense that they can smell people from a distance, but that's a bit too accurate.
aliens can see synthetics
different environments, though they could have done it better in my opinion
different vision of the same area, you would often run into places you've been with another species in singleplayer, and multiplayer maps don't play quite the same with every species either
and that's all I can think of right now. If anyone has something to add or disagrees, feel free to say something