If you play a hunter at all, you've probably noticed a change to pets' Defensive behavior since patch 3.0.2. Ever since the dawn of time, or at least since I started playing (my first character was a hunter), pets on Defensive mode will attack anything that attacks you, the hunter (barring bugs). In this new world, though, pets attack anything that attacks you and anything that you attack. The idea here was just to make things a little more convenient for the hunter; typically, if you're attacking something, you want your pet to attack it as well. And if not, you can use Passive.
It's a nice idea, but it's not been entirely popular. Part of the resistance comes from the fact that it's hard to switch off years of habit. Hunter's Mark, pet attack, I attack. That's how I've done it forever. And part of it is that players want more control; there's no longer any way to attack without your pet attacking if your pet is in defensive. It also really doesn't save much work; a single keystroke per fight, at most (many players have /petattack macroed to things like Hunter's Mark anyway).
Blizzard has noticed that many Hunters have not been happy about this change to Defensive, so they're taking a look at it. If you have any feedback on the new Defensive mode, such feedback is being collected in this thread on the official forums, so you might want to drop by and let them know what you think. Personally, I don't love it, but it might just be fear of change. Skimming through the comments I do seem to be in the minority; most hunters report being quite happy with it. What do you think?