I hate to say it, but power weapons. The teams that holds the power weapons will win most of the time. It still requires some skill though, spamming the DMR or Pistol without pacing your shots will get your a$$ kicked. Get good with grenades; also, Practice. Practice. Practice.
Find out where all the power weapons spawn and sprint to the closest one. The Easiest way to find where the power weapons are on a level to get a guide or watch your recent matches in the theatre, find out who has the weapons and rewind to find where they are.
Teamwork. Teamwork. Teamwork during objective games is also crucial to pwning.