I would recommened that you firstly submit your forum up for a review and get some vital critique from the vb community. There's never been a post were I have asked for some suggestions and got bad advice, you'll defientely get some good suggestions from the .org community.
My own personal opinion is that your style looks far to defaultish. Try to stay clear of the blue variant you've opted for, maybe try another blue variation. Infact, checking back on your style (gradients). Your still using the default images and gradients which is the reason it looks plain.
Your sub forums look awfully crammed together but submitting your forum you'll gain far more indepth feedback on all aspects from other stylists than what I can give.
With regards to the folders the right angles sharp edges were intentional but was always thinking of releasing (designing them first of course) a rounded edge version(s). Now that you've pointed this out I'll probably be releasing a softer more rounded edge variation of this set.