Awesome replayability. There are several categories of your skill and other than a couple of levels, it's possible to kill ONLY the target(s) and nothing else - no witnesses, no clue. Or you can go in shooting.
What I like about it is that unlike most FPS games, the weapons are realistic. I'd love to see a version of Halflife where you could use items like this, for instance. Dozens of small arms, improvised weapons - like the aforementioned shears and a nailgun(which confuses the press to no end!) No bullet holes.
Also, the weapons are useful. There are several that you try hard to get and keep in the game, like a silenced pistol and the nailgun - they make the next levels much more interesting and fun.
Then there's the nine hundred ways to kill people by making it look like an accident. It almost makes me a bit wary - there are so many ways to get killed in most houses. Heh. There are a couple of ways to kill people in the game that are spectacular enough that you end up trying to get in a position to see it happen. The previous games didn't have this.
And then there's the easter eggs. People say or do things sometimes in raction to what you are doing - including a several of them who create nasty surprizes for you.
My only gripes are that the AI is slow-witted and the requirements are brutal. It's MUCH harder, though, to keep them from coming after you, though, I have to admit. Most government agents have a headset that takes like .5 second to yell for assistance into.
Also, the reviewer mentioned the AI being not suspicious enough. On the hardest level, it plays EXACTLY like the older games. You not only have to wear a disguise, you have to stay half a room away from members of the same group. The stealth meter is twitchy and a firefight is usually required. I doubt that it is possible to get a perfect success ratio(ghost or specialist) on the maximum difficulty.