Looks nice and clean overall, and you've kept it simple.
I agree about changing the default avatars but don't like the bright yellow suggestion. Why not keep it grey still but have an outline or silhouette of a fish? Or bubbles? Or anything fish related.
I think your logo is excellent, but is there anything you can do with the empty space to the right of it?
This is just a personal opinion, but on Xenforo I don't like it when you put stats and location back in under the avatar, this goes against the whole point of removing them (so you get a clean, streamlined page that concentrates on post content, not stats, which you can still view on the member card). For example on this page, you have several one line replies, but so much gap inbetween because of the expanded avatar area:
http://www.exoticfishforum.com/threads/hey-im-new.5858/
Try taking those stats out so you just have the avatar, name and rank, then see how much better it looks
Would you benefit from a fish-themed node icon, rather than "NEW" that you have now?