Holy hell I HATE when they throw in unneeded stealth into a game, or force you to use stealth. Snooping and pooping isn't acceptable in every game, and while every game doesn't feature it, there are entirely too many that do, and often without good reason.
Splinter Cell has been a thorn in my left butt cheek for a while because of how it forces you to use stealth, even to the point where you CAN'T EVEN KILL SOMEONE.
Granted, the run 'n gun genre is a bit overdone, but occasionally I prefer to spray 'n pray over having to hide behind a box in the shadows for a half an hour before strangling some dingleberry, then getting noticed by someone across the map! There are plenty of flash games that require you to only use stealth, never kill anybody, and make it to a certain point on a map to continue. If you get noticed, you get bum-rushed by everyone on the map.
I didn't like Arkham Asylum because of the level of stealth that was required. Ok, swooping down from the rafters looked cool in the movies, but totally sucks when you are doing it in real time because of how long you have to wait for just the right conditions before jumping down to break some fool's neck and then quickly zipping away before you get noticed. That sucked for me... mostly because I stunk at it. I always thought I had a good opportunity to snap some spines, but I always got caught by another guy and gunned down in short order. Kinda made me hate Batman for a while.
I don't mind a game with a decent cover system... I don't really see much of that in stealth games. Granted, Splinter Cell does feature this, and it has come in handy. But to combine stealth and cover... it almost blurs the line between which is which sometimes, and can actually make playing the game more complicated when the system can't tell which you are trying to do. I feel there should just be two positions behind a cover: either the left or the right. None of this waiting in the middle crap because it usually takes that half a beat before the system realizes you want to peek over the top to shoot someone, and often that is all it takes for them to run around your cover and shoot you in the head while your character is stuck squatting behind a box looking like a sitting duck.
I like Hitman because at least you can kill anybody and everybody who is or appears to be a threat. You can duck into an alley or a room to avoid them or surprise them when they come looking for you. Why this isn't more of a consistent system is beyond me.