The most significant change was already leaked to WoW Insider a few weeks ago, that being the change to hunter itemization that sees 1 point of agility give 1 point of attack power rather then the previous 2.
Here are the rest of the changes straight from Kalgan's post:
1. Traps are now on a 30 sec cooldown, but can be used in combat (you no longer need to feign/trap).
2. When feign death is resisted, you are still reduced to 0 threat for any targets that did not resist feign death (although if any creature resists the feign, you won't be taken completely out of combat, you'll still be on the creature group's target list).
3. Attack Power and Crit related changes:
a. Hunters now get 1 AP per Agility rather than 2. This change helps us provide more interesting stat distributions on hunter items. Previously, Hunters valued Agi significantly above any other stat, such that it was extremely difficult for us to create variety among Hunter items. For example, before this change, we could have spent 1 stat point on an item to give you either 2 AP or 1 Agi (which gave 2 AP along with a small bonus to crit and a small bonus to dodge... clearly the better choice). However, several changes have been made along with this change in order to offset the AP loss.
b. Hunters now get significantly more crit per point of Agility than before (about 33agi/crit at level 60).
c. Hunters now get more dodge per point of Agility than before (about 20agi/dodge at lvl 60).
d. Ranged weapons have had their damage values significantly increased (and consequently the weapon's dps)... this also helps make weapon upgrades feel more meaningful for Hunters than it did previously (along with the other changes, this change is retroactive).
e. Most Hunter equipment has been updated to have a better distribution between +Attack Power bonuses and Agility.
4. Aimed Shot related changes:
a. Aimed Shot now resets the Hunter's shot timer. This talent was always intended to be an "opening" shot, and not something that would be woven between shots. However, since Aimed Shot became such a core element for Hunter DPS, we didn't want to make this change until we were prepared to make several other changes to increase hunter DPS to offset it. Fortunately, this means that some of the odd side effects of the old Aimed Shot are no longer relevant, such as requiring hunters to use extremely slow weapons to maximize their DPS (weapons slow enough to be able to use Aimed Shot in between auto shots)
b. Arcane Shot now does substantially more damage than it did before, and scales with AP.
c. Steady Shot has been introduced as a core ability in the expansion, and provides much the same kind of gameplay as Aimed Shot did in the past (a shot you weave in between your normal shots), but to a much more managable degree (since it's a 1.5 sec shot, it doesn't require as slow of a weapon to take advantage of, and doesn't require to stand still for as long).
5. Other note: Wyvern Sting may now be used in combat.
EDIT: Kalgan also posted a word about the leaked pet skills. Apparently, as has been suggested before, the rumoured skills have been in the WoW database since launch and are not intended for use. The real pet skills will be released at a later date.