There has been a great deal of change made to all of the different classes with the coming of Cataclysm. Hunters are seeing more than their fair share of them including 3 new abilities that are intended to make them a lot more effective in any situation. Hunters have always gotten the short end of the stick and unless the powers that be nerf what they have done, they may become a very viable part of any raid or in the PvP run you get involved in.
First the new abilities, starting with Cobra Shot that you get at level 81.This shot causes Nature damage instead of pure Physical damage and when combined with the talents in the Beast Mastery tree could easily become the go to shot in any situation, especially when going up against heavily armored targets. It will share a cool down with steady shot.
At level 83 Hunters will get Trap Launcher, this will allow you to shoot any one of your traps up to 40 yards away in a similar fashion to the Freezing Arrow Trap which will be removed. Finally at level 85, Hunters will be given Camouflage, which effectively hides them and stops them from taking any ranged damage.
These are the new abilities, but only a small part of a much bigger picture. Now instead of using mana, Hunters have Focus which builds up like Rage instead of running out with use. In order to use abilities such as Steady or Cobra Shot, Aimed Shot and many more you will need to have enough focus built up. This completely changes the dynamics of playing a Hunter and will take some getting used to.
Every Hunter has had to carry his ammunition with him no matter where he goes, at first they had to carry a quiver or an ammo pouch. This was followed by taking these away and letting the Hunter carry his ammo in a regular bag. It meant being able to carry far more ammunition than ever before. In Cataclysm Hunters will no longer need any type of ammunition as all bows, crossbows and guns will do damage without them.
All Hunters will start with a level one pet and will now be able to carry up to three pets with them at all times. While they have three pets that you can swap out any time, they can still only have one pet out at a time as long as they are not actively involved in combat. Should a Hunter wish to swap out any of the pets he is carrying, he will still have to visit a Stable Master. These are just a few things that have been made to help the Hunter recover much of they lost in WotLK.