Mania is quickly becoming the voice of choice for analysis on Hunter changes, and her latest take on the Growl changes recently released in the patch 2.4.2 notes is no exception. From what Mania can see, Growl's threat isn't scaling with Hunter Attack Power at all, but Blizzard claims it is, so she's chalking her findings in that direction up to faulty data. What she does determine, from both her research and Blizzard's posts on the subject, is that the change here has nothing to do with Hunter's RAP or AP values -- it only has to do with the pet's AP values. Before, if pet AP got a bonus, so did Growl's threat. After 2.4.2, any bonus to pet AP won't affect Growl's threat.
So Hunters who routinely get their pet buffed are probably going to feel this change a little bit (though in most situations where pets get group buffs, I'd imagine Growl would be turned off anyway, in order for the tank to control threat). And Hunters who use boars, of course -- as Daniel explained, that extra AP from Charge doesn't work for Growl any more. Mania doesn't stop there, however -- since we know now that the only thing affecting Growl is the Hunter's Attack Power, she's determined to figure out just how much AP can scale Growl up (she's guessing it starts at 1955, we'll see what she comes up with).
Of course, some players have found that this works pretty well -- especially MM Hunters, who generally have more attack power (thanks to talents which increase it), have no problems with drawing aggro away from their growling pets. Blizzard's reasoning on this may have been solid -- to make the abilities of the pet rely on Hunter stats, not any temporary pet buffs. Hunters with Boars won't get that nice burst any more, but Hunters with a high RAP will do just fine.