Most of the complaints I've heard in reviews are:
The Alien seems to attack without warning and/or from an arbitrary direction.
This means the reviewer wasn't wearing headphones or didn't bother to recognize audio cues for the creature entering/exiting the ceiling vents. It's instincts are perfectly learn-able without it's routines becoming predictable. Sure, things like having to lean backwards when you're inside a locker to prevent it from detecting you may not have been fully communicated to the player, but not realizing things of that nature Mission 5 seems to just be a consequence of people being unwilling to experiment with the mechanics of the sandbox. Almost without fail, small little stealthy maneuvers I used functioned as I would logically expect, and an on screen tutorial wasn't necessary. It's stealth game 101. Mind you, I only hid in one locker for the entire game, it's perfectly possible to hide around the corner of a doorway and move counter to the Alien to stay out of sight. I've seen far too many people assume that the easiest course of action is to hide until the Alien finishes it's ground patrol, it isn't. If you hide for too long, it will probably find you. Move counter to it with a wall between you, moving towards your objective, hide temporarily if needed. If it's patrol is getting annoyingly long, cut to the chase and use your flamethrower to get it into the vents again. Use the window of opportunity to get to your objective without resorting to sprinting.
The save system is unforgiving.
It's an old school difficulty thing that was used equally to or even more effect than in Dead Space.
The game is too long.
I feel that CA gave us about two modern "blockbuster" games worth of content without us having to pay for both or wait for a sequel. I feel like the game was perfectly paced and (almost) every "ending" was proceeded by another for the improvement of the overall package.