The Beast Mastery tree 31 point talent is Bestial Wrath. This is the talent that makes the pet turn red and huge for 15 seconds and do 100% more damage. It's likely that the pet will also Frenzy, increasing attack rate by 30%. You get an enraged pet on you and its like someone dropped you into the lion cage wearing a meat suit. It's not pretty. An enraged pet will chew up a rogue and just about any caster most of the time.
Unfortunately, other players do have some defenses against an enraged pet. Other hunters will Feign Death and the pet will immediately stop attacking and return to its owner. The pet owner will have to retarget and redirect the pet at the feigning hunter, and this uses up a good chunk of the brief time the pet will be enraged.
Mages will blink away from the pet, mana shield and do their ice cube thing if they are specced that way. This pretty much saves their cloth-wearing hides.
Paladins will just use their infernal bubbles and heal themselves. When I was specced for Beastial Wrath I didn't even bother using it against them. Nerf pallies! (Just kidding pallies, don't beat me.)
Priests can bubble and heal too.
Druids will go into bear form to absorb the damage and then heal themselves.
Warriors will just plain outlast the 15 seconds of Bestial Wrath due to their armor and hit points. Nerf warriors!
Warlocks are just plain ornery with their ability to siphon life from foes and (I believe) their pets. Still, an enraged pet can put a good dent into them.
Rogues probably have the toughest time with the pet. They have no way of escaping other than sprinting, and many pets can match their sprint speed. About the only tool they have is Evasion, which should help them avoid damage for the 15 seconds of Bestial Wrath.
Bestial Wrath isn't supposed to be an "IWIN" button but my issues with it are that too many classes have ways of sidestepping my pet when I use this ability, and it's on a two-minute cooldown so I get one chance at it per fight.
The benefits of going with the Bestial Wrath talent is that you spend points in other Beast Mastery talents and end up with a significantly tougher pet as a result. I did try this tree for quite some time and do really like having an uber pet, but ultimately I felt like I was sacrificing too much damage potential for my Hunter on the other talent trees.
The talent is scheduled for a bit of revamp in the upcoming 1.9 patch. The pet damage will be reduced but the length of the wrath will be extended from 15 to 18 seconds.