I rented this and I disagree on some points.
I don't know your reviewing process but I think you underrated the alien campaign, which you said was the one without the memorable and thrilling moments. I had the most fun with the alien campaign, though admittedly I ruined the marine campaign for myself first. Having previously played AvP 2 for the PC and loving the aliens gameplay in that title, I jumped straight into the alien campaign, stopping about 3/4 of the way through to give predator a try. This proved a foolish decision on my part, as spending about 8 hours playing as one of two races that in their respective campaigns tear through Marines like wristblades through butter can make for a very paranoid marine player. True enough the low lighting and claustrophobic spaces combined with the iconic motion tracker sound provided a nice creepily intense atmosphere, I was constantly concerned an alien or predator would drop from the ceiling and pull me through the vents, or any number of instant deaths I had seen or in some cases performed in the other campaigns. I guess I mean to say that I had so much fun playing the other two stories and they did their job wll at creating an intense experience for the marine that I stopped playing when I forgot I was running on protagonist power, making me immune to the usual cinematic insta-deaths.
But I've never had so much fun playing alien. I haven't played AvP 1, but in AvP 2, the grand majority of the stealth portion was the facehugger level (which I was dissapointed to see was absent, it made multiplayer more interesting if you could get people to turn the option on) and any segment when you had to sneak past a sentry gun or a few synthetics. AvP 3 (I noticed it's not called AvP 3, and I assume there is a good reason behind this, however I'm going to call it that here for the sake of clarity) encourages stealth the entire time. After I got used to their rework of the wall-crawling controls I loved it. True there are segments wherein you are breaking loose or you have no stealth option (or messed that option up) but the best part about those is that they don't get in the way, they are managable and fairly easy to get through or reload and try the stealth again. But those stealthy moments are the ones I remember. I remember knocking out the lights and impaling a man just as he uttered the words "Just another blown fuse." I remember crawling across the ceiling of a warehouse, dropping down and picking off an entire squad of marines, they only caught glimpses of me as the lights flickered off or as I leapt back to the ceiling after biting a hole in their comrade's head. I remember screeching from the vents, daring the marines to investigate as I picked them off, each time passing a lit-gas canister, wondering how best to utilize it, only to have the last marine spray the ventilation shaft with fire. From a safe distance deeper in the vents, I thought what to do next, remembering the gas can and thinking: "Wait, I wonder if that'll set off the explosi-HAHAHA! What an idiot!" as he detonated himself with exceptional comedic timing. I remember slaughtering a team of 5 Marine players and one Predator in an online species deathmatch with the help of only one other alien, creating panic in the team that had performed well together in previous rounds, contributing to a win for the aliens with a 20 point lead.
I remember the aliens gameplay quite fondly. I think you should give it another chance. However I respect your opinions and I realize not everyone enjoys playing Alien, it was true and AvP 2 and its true in AvP 3. It's even more challenging now that marine players have finally learned to look up, something they were comically lax about doing in AvP 2.
Other than that I agree with you. At first your suggestion of a cover system seemed like a bad idea to me, but now I'm curious. Just what kind of cover system were you hoping to implement against foes who can walk on walls, see in the dark and leap to the tops of two-story pillars, or can see your location every time you move? It would have to be a really good cover system to work against all that at once.