Please take my comments in the spirit in which they're intended, and don't feel the need to explain why you didn't do every little thing: I'm just throwing out the benefits of my own experience, and, it being your house, you - joy! - get to decide if you think they're good ideas for your board or not.
The limit of 20 threads per page on forumdisplay.php is reasonable, given the multi-line thread entries, but I personally find 10 posts per page on showthread.php to be incredibly, powerfully irritating.
Take a look at any post on the board. Okay, see the date/timestamp? Directly under it are two horizontal rules, where one is necessary. It's governed by this code:
<div class="spacer"></div>
<div class="hr" style="width: 100%;">
<hr />
</div>
<div class="spacer"></div>
<div class="spacer"></div>
<div class="hr" style="width: 100%;">
<hr />
</div>
Turning this into normal people code, we get:
Which is just plain silly, of course. I'm not sure if, under a different set of circumstances, those spacer divs might get populated with something, but right now that code drops a bunch of extra divs [waste code] and two horizontal rules [waste space]. Is no good.
The "Go to first new post" icon to the left of the thread title on forumdisplay.php is small enough to be difficult to hit. The multi-page icon is 12px high, so increasing the first new post icon to 12px in height wouldn't distort the layout; more width would make it even easier to hit. Presumably, this is a link often used, and making it the smallest layout element kind of sucks.
On the front page, the description text is allowed to trail all the way across the page until it hits the "Go to Last Post" image [itself utterly useless, since this board gets so much traffic], making it a bit of a visual mess. Better to make it only 75 percent of the cell it's in.