The MyBB Humanization Project

[Yumi]

I like how you're removing crap - I've generally thought there to be too much anyway.
You'll probably get plenty of positive feedback, so I'll just focus on my criticisms:

• How well does the signup page work with Javascript disabled? In particular, I suspect Javascript is being used to display the "Username", "Password" etc text, but what happens when it's disabled? Only other method I can think of is HTML5 placeholders, which isn't supported in any stable version of IE anyway. Actually, it may be possible with hacky CSS, but I wouldn't be so sure IE supports that either.
  
• The 'error: log-in' page partially serves the purpose of notifying users that they need to log in. If I click on a link to a thread then a login page comes up, it can be confusing - at least having a message clarifies why I cannot see the thread. A short message should be suffice - no need for all the bullet points (though the bullet points in this post is required)
  
• Paging links - MyBB's suck in the forumdisplay, but you should make them bigger. Not everyone has perfect skills with a mouse, so make the click target larger. Especially useful for people browsing on touchscreens
  
• Your new thread seems to favour widescreen monitors, as you've mentioned - true that they're becomming commonplace (though not universally, eg iPad is 4:3) but not everyone has their browser fullscreen'd
  
• MyBB's standard for submit buttons is to have them below the form and centered. I suppose you've switched them everywhere to be on the right column (except profile page)? Interesting to note is that the new reply, it's aligned to right-bottom, whilst in quick reply, it's right-top. Not a UX concern, but just being anal about consistency.
  

• The postbit controls are faded out until you hover over them

  Just note that hover effects aren't viable with touchscreens
  
• It's interesting that you changed a "single column" layout to "two column" for your new thread page, but did the opposite for the profile page. Change for the sake of change?
  
• You've got yet another layout for the private messaging. I would consider that PMs should resemble the forumdisplay layout, but you've got the new thread button on the top-right, but new PM button on the top-left. The author/subject is a little confusing as there's no obvious distinction between the two. I personally prefer your forumdisplay layout.
  
• Stripping out cruft is fun, yes, but some things that may seem useless could be nice to keep. Stuff like thread view counts may be mostly pointless, but can provide interesting statistics for users (not just the topic creator), just like images, colours etc. Not really a UX issue, and different people will have different tastes, but perhaps something to keep in mind.
  

Hope that gives some ideas.