What it means is that, currently, Growl scales with the Pet's AP. Where this has had the most massive advantage is in the use of the boar pet: Because a boar's Charge ability adds a massive amount of AP to the boar's next attack, it allowed a boar to cast a super-powered Growl directly after they charge, resulting into massive threat gain.
Thus, these changes are a double nerf to boars. Since Growl is casting at random, they were already losing that massive threat gain, but now that loss of threat gain is being made official. Not only can Growl no longer be cast right after Charge, but it will no longer gain extra threat from the AP buff.
The fallout is already being seen in the Hunter community. I myself already know quite a few Hunters who are permanently stabling or abandoning their boars. I myself am still clinging obstinately to mine, but I have to admit that that new Dire Raven pet is starting to look pretty cool. I leveled a Tauren Hunter with a Swoop, so I know those birds are good pets too, and Screech is amazing for aggro generation.
Some are theorizing that this nerf to boars was meant to encourage Hunters to drop their boars and take a wider selection of pets. I'm not sure this will be the effect at all. Instead, you'll simply see Hunters returning to ravagers and cats for maximum PvE dps, or scorpids for maximum PvP utility. If they really want to encourage pet diversity, they need to add new skills to some of the more neglected families and leave poor boars alone.
However, before we get too mad at Blizzard, we should check out the news coming in from the PTR: Some Hunters who have done testing on the Growl change have said that it may be an overall buff. As long as they have a decent amount of AP, they're finding it easier to keep aggro on test now. Zixi of the Alterac Mountains server posted a video, seen above, in which she was able to stay below aggro on a level 67 pet while Survival specced rather easily. There's probably more testing that needs to be done before we can give a final verdict on the change, but what could be a big hit for boar pets could still be a win for Hunters overall. Just keep your AP higher than your pet's AP (and higher than 1200 in any case), and you could find your solo career getting a lot easier.
What's also nice is that no matter how this whole Growl thing shakes out, there's still going to be a few definite buffs for Hunters in this patch. We've already covered the Scare Beast change, and there's also the change to Aspect of the Viper. It will now grant 35% of your level in mana regen every 5 seconds, which will translate to 24.5 mana every 5 seconds at level 70. Since Hunters have been having some mana issues since 2.4 with the new spirit-based regen system, this should be helpful to counter some of that. On the other hand, it doesn't seem to be a complete solution, since you'll have to actually have Aspect of the Viper activated to take advantage of it.
Overall, this should be a very interesting patch for Hunters if these changes make it to the final live version of the patch. Despite the heavy hit to our poor beloved boar pets, if the preliminary reports from the PTR prove true, this could be a very good patch for Hunters that will hopefully close our arena ranking discrepancies, help a bit with our mana and pet threat issues, and put us into a decent position headed into Wrath of the Lich King.