Everything about grenades honestly has put me off most FPS games, especially in online play. I work best when I'm at a safe distance (e.g. I'm on the other side of the map and looking at you through a sniper scope), but when someone has played an FPS way too damn much and has learned how to lob a grenade perfectly into your lap from three stories below you, part of what made the game any fun dies a little bit.
I understand grenades are a classic way to wage war, but an FPS can be buckets of fun without the stupid things. Team Fortress 2 is quite frankly hilarious, and although I do have to keep an eye out for the occasional spy knifing me in the head while I'm zoomed in, I can play the game without worrying that the guy on the "wrong" end of my scope can't end me with the same speed and accuracy as I can end him.