Unprecedented platformer. Indeed, it raised the bar on puzzle side-scrollers.
Gameplay is awesome. The AI, although high, can still be thwarted by a crafty player. Sure you can sneak around here, shoot there, etc, but how about throwing stones to trip motion-sensor turrets to do your dirty work for you, or distracting a guard before sneaking up and pistol-whipping them in the back of the head?
Also, it expanded on the classic "retrieve-to-advance" game feature, which involves finding an item that allows you to progress in the game or earning credits to gain access to whatever. The errands/items the character had to complete/retrieve are reminiscent of RPGs. And the character himself is very human, which i think helps engulf the player more into the game.
Difficulty is medium-high, since, being very human, he's very quick to die. Get shot, you're dead. Fall too far, you're dead. Although challenging, it doesn't deter from the quality of the game since who wants a game that's too easy? Besides, this is a game where brains are more valuable than brawn, so use your head.
The plot itself is very submersive. From what i recall, the character is trying to remember who he is and why everyone is out to get him after suffering a bit of amnesia. Slowly realizing his significance and who the real bad guys are while running around a futuristic world with a single pistol as his only trusted friend is definitely a wild ride.