In general, I really like the way these feats work. I like that each bending discipline gets a different benefit, and I support the idea of keeping the benefits tied closely to the two bending disciplines involved. In general, good work. I think these are the best of the different incarnations of the “Bending Knowledge” feats. That said, I’ve been thinking about these feats for a while, and have a few comments:

1. I think that the prerequisite skill ranks should increase to at least 9. The presentation of these types of abilities in the show indicates (at least to me) a high degree of proficiency with bending and substantial study of the bending disciplines. 3rd level seems a bit to low to me.
2. I like the Air and Fire abilities granted by the Waterbending Study feat, but I can’t think of an example from the show wherein the Earth ability has been used. The only example I can think of is this earthbender (shown here in the act of tunneling), but this seems like a refinement of the Excavate seed rather than a new ability and there’s no evidence linking him to any significant knowledge of waterbending. However, I’ve been wracking my brain to think of an alternative ability, and I can’t think of any. But nevertheless, I think we should try to find an alternative ability. I’m also uncomfortable with the waterbender’s swim speed ability, but that’s a different debate.
3. I like Airbending Study. No significant issues here.
4. Firebending Study seems like the hardest to characterize, mainly because we’ve never seen any non-firebenders learn anything about firebending. That may change with the next few episodes. I do have a few comments though. First, we should clarify what a “partial defensive action” is for the Air ability. Does that mean making at least one Deflect Attack or is it something else? Second, I’d say that the Water ability should make half of the blast’s damage fire damage (steam anyone?) instead of actually adding damage. Automatically adding the bender’s Wisdom modifier to all blast damage rolls seems a bit much for the defensively-focused waterbender. I like the Earth ability, though. It seems very much in keeping with hasty earthbenders like Shin Fu.
5. I like the Earthbending Study feat, especially the Air ability (which you can see when Aang fights the Sabre-Toothed Moose-Lion in Bitter Work) and the Water ability (which is just downright useful). However, I can’t think of any examples of the Fire ability in the show. Again, I don’t really have any replacement suggestions off the top of my head, but nevertheless…

So, those are my suggestions. Overall, I like the feats. Good work.

Now, to address some related issues that I arose in this post.

First, I still can’t find any examples from the show that support giving waterbenders a swim speed. I hereby recommend removing the Water Walk seed and the waterbender’s swim speed and giving waterbenders a Ski/Water Walk ability as part of their class, increasing the speed as usual. However, if someone can post a screenshot or point me to an episode wherein a waterbender swims with the ease that a swim speed implies, I’ll definitely reconsider.

Second, to address the issue of tunneling Earthbenders I raised above, I suggest we add the following to the Excavate seed:

Burrow: By bending earth into a small tunnel, an earthbender can burrow through stone and soil. As a move action, an earthbender can move up to his base speed through any substance he could bend. This form does not give the bender any ability to see through the earth or sense what is occurring outside his tunnel, though other seeds could be used for this purpose. Earth displaced using this form is moved to the periphery of the tunnel, making a bulge if the tunnel moves within 5 feet of the surface. The tunnel created using this form collapses after the earthbender passes through, and creatures without burrow speeds cannot use the tunnels. For every 5 by which an earthbender’s skill check exceeds the base DC, the earthbender’s burrow speed increases by 5 feet.

Or something like that. What do you think?