So after trying the 3 specs for marauder (Annihilation, Carnage, Rage) I have decided to mak a short guide on it. Note that this is from a PvP standpoint!
Carnage: I personally found Carnage the easiest to use and the most effective. Force Camo is a skill that will cloak you for 4 seconds and reduce movement speed by 30% during the time. Also all dmg is reduced by 50%. The Carnage spec is not "focused" on this ability but the main Idea is to run in with force charge, Kite(Don't stop running while you attack. This causes them to have a hard time focusing) the player you're attacking, and use all of your skills. When you begin to lose health about 25% left activate cloak of pain and blade ward. This will reduce dmg like crazy while you continue dmging then when you are about to die use force Camo and retreat. This works extremely well because Carnage does crazy burst damage but without a good healer on you you will die fast.
Annihilation: This one I like second best. The Annihilation tree offers less burst dmg then the Carnage tree but it also has WAY better survivability. The tree offers a self heals when you are bleeding. Many of the abilities here are mainly focused on Force Camo and bleeds. While the Carnage tree bursts in with heavy immediate dmg the annihilation tree slows and stacks many bleeds at a time slowly dmging the player you are attacking. Don't take me wrong annihilation offers alot of immediate dmg as well but not nearly as much as carnage. In a long battle the bleeds will add up and end up doing huge ammounts of dmg.
Rage: I tried this spec for 2 warzones and I switched back to carnage. I found it just lacked dmg. I don't want to talk bad about ppl's spec so I will just leave it at, Some could make it work but i can't.