Alright, I'm officially rolling on this.
I've attached my first pass which is focused on tightening up the layout, fleshing out the visual hierarchy and (hopefully) addressing some usability issues. The mockup is of the main discussion display, as this is the most involved display and most of the changes are likely to trickle down. It's done in greyscale with no supporting imagery for now. In other words, it's super dull. Once we have the layout nailed down, I can begin to introduce the actual theme and presentation: supporting graphics, colour, font streamlining, etc.
A second version of the mockup includes some numbered markers for reference. Here are some quick thoughts re: the rationale behind some of these changes:
1) User menu. Essentially identical, but with the right alignment corrected to correspond with the forum margins.
2) Banner area. I've been pretty generous in allotting myself space to work with. Expect this to condense once we begin introducing the actual imagery.
3) Main site navigation. I wanted to beef this up a bit, not only to improve the visual hierarchy but to anchor the layout. Also, I'm using an "Active" state on the menu items as an indicator of current location. This would work to replace the giant <H1> currently in use (i.e. the large text that reads "Discussion Forum"), buying us some valuable real estate.
4) Search field. I noticed that the site search also pulls up forum results. As such, are both current fields needed? I've tucked a single search field into the main nav in the assumption that it is not. Let me know.
5) Second-tier nav. I'm playing with the idea of moving the current forum nav up into this area. This would help to condense things a bit vertically and also to consolidate them into related areas.
6) "Bread Crumb" navigation and thread title. Instead of a single "back" button that steps up the forum structure one click at a time. Not sure how doable this is, but it's a great deal more usable.
7) Thread-related functions. First, I've dumped the "View" buttons altogether. Here's my reasoning: the "tree" view is inherently broken, and collapsing the discussion tree (currently the little green arrow pointing right) in "combination" view essentially gives you "flat" view. So why not just default to combination view with an option to toggle the discussion tree? I've also replaced "Post New Thread" here with "Recent Posts," as this just made more sense to me and (I wager) is likely to see more use. These buttons will probably need to move a bit to accommodate longer thread titles.
8) Discussion tree display. I've moved the collapse button to the upper-right and given it what I feel to be a more appropriate icon. Also included table headers.
9) Post content. Not much to say here but I've included a note marker for related feedback.
Let me know where I've been too liberal with the machete, have introduced changes that will be difficult to implement, or have simply lost the plot altogether. I'll take that feedback and do another pass with those revisions in mind. I've got some neat ideas for the theme itself, I think, and am anxious to get on to the fun stuff!