The one thing about this game that irritates me is the inclusion of enemies that basically all your gear is ineffective against, as well as mark and execute.
I mean, I get it, you don't want me using mark and execute as a crutch to make the game too easy. I gotcha. But I still have to silently take out bad guys to even earn it... if I've performed well enough to earn the skill, let me fucking use it. Don't put dudes with helmets that pop-off when I head-shot in there so that I'm automatically noticed and punished for using a feature central to your own damn game.
Or at the VERY least, let me select an enemy twice (as opposed to say, using three marks on three different targets, I could use two marks on one dude with a helmet, and one on another guy), so that I can double-tap him.
Also, the armored guys immune to sleeping gas, shock mines, etc? Fuck you. What the hell? If I want to slowly crouch-walk behind a guy so he doesn't notice me and subdue him, or jump on him to subdue him, fine, I'll do that. But if I don't want to do that, then let me use any number of perfectly cool weapons and gadgets to sneakily take him out.
It's fucking jarring for a game that gives you so much freedom for 90% of the gameplay to suddenly put up a wall and says 'NO! You must complete this particular challenge exactly as we say'.
The end result of this? Mastering Panther on Realistic difficulty by upgrading the ACR4000 (or whatever it's called) and just burst-firing dudes in the head from cover for silent kills. Least fun way to do it, but at least you know it works every goddamned time on every goddamned enemy.
/rant
*inhales*
Now, all that said... I actually really enjoyed the game. I particularly liked the way you can play through the missions in different ways. Lots of great challenge and replayability from that.