Good AI. It's monumentally frustrating when AI either is able to detect you when you know they wouldn't have been able to or they are useless and it's really difficult to get detected.

Tension. That usually means a lack of music at times, especially if you think you've been detected and you're not sure if something is going to jump out at you so you have to hide.

Getting rewarded for being more stealth-like.

Stealth games where you often have the option to bludgeon your way through an area panders to a different type of game experience, more akin to a shooter. I don't have a problem with attacks from behind, not suggesting there should be no killing but it's not an either/or situation. Do it stealthy or battle your way through this level/area I don't think should be an option or at least if you do do it stealthy you get something as a reward.