Just finished the first two marine levels on hard. Not bad at all but from what I read these are the two best so I expect the rest to drop in quality.
More random details:
Aliens move slowly but they seem to do that only when you haven't shot them yet, else if they know that you detected them they will start running and jumping around; that's reminiscent of when they invade the barricade through the ceiling in Aliens. They make good use of the environment and it can be hard to track them in the dark because they keep moving. They are also pretty tough, at least on hard difficulty, and while they go down easy with the shotgun it can take a few bullets with the pulse rifle; headshots do more damage but that can be hard to do in some situations.
Acid spray is satisfyingly nasty. Getting some alien body part exploding at close range can easily get you from full to low health, or even kill you right away.
Aliens adopting a blocking stance in front of you looks stupid, especially when you pump them full of bullets. I assume that this protects them only from melee hits and not weapons. Speaking of melee, that move is mostly done in order to push an alien back and stun him for a second, which can be enough to save your life, else it doesn't seem to deal a lot of damage if at all.
Facehuggers approach moderately slow and often pause, giving you time to pick them off. So far they are pretty easy.
It seems that aliens may spawn at different points, which is pretty cool. For exemple at some point I headed toward an elevator and before reaching it an alien would break through the floor and climb towards me. I died a bit later and reloaded to the last checkpoint located before that elevator. When I came back I expected to see the alien jump at me from the floor again but this time it broke down through a vent in the ceiling and surprised me! I haven't had to repeat long portions so I can't tell if this is consistent but that's a really nice detail that can keep some surprise if you decide to replay the game, or even if you have to reload at a previous checkpoint.
Story is mostly ancillary. You play yet another rookie — and that's even how he is referred to — that manages to survive far longer than any other experienced soldier and you are guided by a female who tells you what to do. I know that I have seen that pattern in quite a few FPS of the last few years. No interesting character so far, so it's hard to care much when one of them dies. You can find a lot of audio logs, which are the collectible tokens of the game, often placed where they do not make sense, but they do not tell anything interesting, just the usual thing about people who discover ancient artefacts and then hide from the monsters that have been unleashed. System Shock 2 it ain't.
Items have a fucking glow over them so you can easily spot them. That feels somewhat insulting. I can accept that they get a small light on them to helps you locate them, and I think that Dead Space did that, but that glow is just ridiculous.
So yeah, for the most part I'm pretty positive so far. But then again that's only two levels and I will see how it fares. I also haven't played as predator and alien yet and I remember that those parts were weaker when compared to the marine's.