First, the pet resist skills have been removed completely. Instead of gaining the old trainable resists as they level, pets now simply gain a flat +1 resist every level. This is pretty certainly an overall nerf, especially since there's no way to get the previous max resist level of 140. Some Hunters are already asking for 1.5 resists per level so that their pets can at least somewhat approach the old levels. It may be, however, that Blizzard means for you to set aside 3 talent points if you really want some extra protection against the elements for your pet.
The most controversial change to go in with this patch is a complete revamp to how Kill Command works. Now, instead of a quick cooldown damage attack that lights up upon a Hunter crit, it's a 2 minute cooldown triggered ability that causes the next 3 pet special abilities to hit for extra damage -- 60% on the first, 40% on the second, 20% on the third.
We were told by Koraa that Kill Command would be changed to allow us to have better burst damage, but it does honestly look like like nothing more than a massive nerf. Considering a Beastmastery Hunter with good critical strike percentage and the Focused Fire talent could spit out an extra 500-1000 damage every 6 seconds or so using Kill Command, it's hard to see how this new version is better. Even if it is controlled burst damage, the burst damage itself probably won't be as much as the old Kill Command, given that pet special abilities don't hit for much. As it is, most Beastmastery Hunters will probably save Kill Command for a Cobra Strikes proc anyway.
Of course, reports from higher level hunters are that rank 2 Kill Command works the same as before, which means that the rank 1 Kill Command may be a bug or a mislabeled ability
If it's not, and rank 1is the "real" ability, why Kill Command was nerfed so massively is unclear. Some Hunters believe that we're just getting too powerful, and they wanted to nerf our DPS some way or shift DPS emphasis away from the Beastmastery tree and onto the others, while others believe that Blizzard simply doesn't realize how badly they nerfed the ability. Either way, this Hunter is hoping they reconsider, or at least fiddle with it a bit more. In the meantime, I'll just be over here, testing with my worm.