This is a pure copy and paste from the official wow forums. I had nothing to do with this post and full credit for putting it together goes to Simca (Blood Elf Pally)
Last Updated: May 19th, 2013 - All changes are in this post now.
Major Pet Changes
•Blighted Squirrel is now an Undead instead of a Critter.
•Celestial Dragon's Breath ability has been replaced by Flamethrower.
•Ruby Sapling, Teldrassil Sproutling, and Wither's ability lineups have been greatly changed: Poisoned Branch, Thorns, and Photosynthesis have shuffled positions.
Major Ability Changes
•Impale – Cooldown is now 2 turns, down from 4. (PTR Patch Notes say 3 turns, but that is wrong.) Now deals double damage below 25% instead of instantly killing the enemy.
•The version of Flamethrower used by the Essence of Competition and the Spirit of Competition now has no cooldown.
•Predatory Strike – Now deals double damage below 25% instead of instantly killing the enemy.
•Sticky Grenade – This damage cannot be dodged, blocked, or avoided.
Minor Ability Changes
•Amplify Magic and Ancient Blessing now specify they affect the "active pet" only instead of the "entire team".
•Darkflame now uses a synonym of the word "burn" instead of the word itself to avoid confusion with the Burn status effect.
•Dreadful Breath now specifies "Cleansing Rain" instead of "rain".
•Interrupting Gaze, Interrupting Jolt, and Kick – Tooltip wording changed from "interrupts multi-round attacks" to "Interrupts the opponent's round if the user goes first".
•Stun Seed – Tooltip wording changed from "enemy" to "current enemy pet".
•The "Blinded" status text color is now Purple. The status effect colors for all abilities using Blind have been changed to the new color.
•The "Webbed" status text color is now White. The status effect colors for all abilities using Webs have been changed to the new color.
Minor Pet Changes
•Tiny Red Dragon's icon is now red.
•Emerald Whelpling's description had a missing word added ("dragon").
•Lucky Quillen Cub's description now states "Offspring of the Imperial Quillen" instead of "Offspring of the winged guardian".
•Murkimus the Gladiator's description now states "And for a time, the only one" instead of "And the only one".
•Shimmershell Snail's description had "Felwood" changed to "Darkshore".
•Baneling's source had the "C" in "Starcraft" capitalized.
•"Starcraft 2" was changed to "StarCraft II" in Mini Thor's source information.
•Murkimus the Gladiator's source has been changed from "Promotion: Arena Tournament 2009" to "Promotion: Arena Tournament".
•Zone Corrections (Finally): Albino Snake, Dark Phoenix Hatchling, Armadillo Pup, Guild Page (Alliance), Guild Page (Horde), Guild Herald (Alliance), Guild Herald (Horde), Lil' Tarecgosa, Lunar Lantern, Festival Lantern, Rabbit, Squirrel, Prairie Dog, Snake, Spider, Mountain Cottontail, Black Rat, Strand Crab, Beetle, Spider, Rat, Small Frog, Roach, Fawn, Larva, Robo-Chick, Rabid Nut Varmint 5000, Frog, Gilnean Raven, Adder, Chicken, Biletoad, Horned Lizard, Stormwind Rat, Highlands Skunk, Malayan Quillrat, Skunk, Black Rat, Strand Crab, Rat, Small Frog, Toad, Roach, Maggot, Cat, Chicken, Fluxfire Feline, and Jade Crane Chick.
"Even when you are winning soundly, you must always give your opponent a way to retreat with honor. If you don't, then he has no reason to surrender. He will fight until the bitter end, and you will pay a larger price for victory."