I thought the first rule would be aiming.
Serious note, snipers do make hardcore unplayable boring. No matter what map it's just a camping, c4 fest. Most of them can't play for shit (and I'm not any good either, so that makes it worse). They just wait for you to wander into the open (and, nobody ever does because everybody knows where the sniping alleys are) or stumble into the c4. Yawn.
More importantly, I hate those fucking faggots who refuse to play the game mode. It's domination, not sit in the fucking window of the obscure corner of the map and wait for someone to wander into view. It's one thing when they are defending a point (or the flag, bomb site ect..), that's fine. It's the "snipers" who lock down a meaningless portion of the map, fucking over their teammates.
If your sniping to actually, you know, hold a key point on the map that's fine. No issue. That's about 1 in 20. Most "snipers" don't do anything during a game, unless they find a spawn point and get a few cheap kills. Mostly they spend the entire game sitting in a corner waiting for you to wander into view...making my game boring. Especially since they miss and get pissed when you noob tube them off the planet.