Wrath of the Lich King is going to do some interesting things to pets, especially in terms of leveling your pet and loyalty ratings.

Big Red Kitty has the story. WotLK Beta, Pet Winners and Losers

For example:

Bats Sonic Blast. What makes this two-second stun better than everything other pets’ is the uber-short cooldown. If you’re mana-draining hunter in a 5v5 team and your primary target is the paladin, it’ll be hard to choose a better pet than a Bat to interupt that paladin’s heals.

So if you play a Hunter, or you’re being played by one, or you want to know about that odd pet that’s going to be attacking you in the BGs, or you want to know more about the new pet leveling system, then check it out.