Exotics don't make a huge difference; but having good legendary weapons certainly does. They just generally have larger magazines, better stability, etc. (and of course more perks) than their commoner counterparts. If you like autorifles, I highly recommend picking up either Up For Anything from the vanguard vendor, or Vanquisher VIII from New Monarchy. Both are excellent. More important than the weapons you use, however, is knowing general principles for Crucible success. Always watch the radar; never go into a confrontation where you're outnumbered; always pick up the Heavy Ammo (but be courteous enough to wait for teammates to get it too). Etc. And don't be afraid to die as you experiment and find a setup that works for you. For example, a lot of people swear by shotguns; but I just can't seem to use them correctly and do much better with a Fusion Rifle special.