Well, I realize this is going to be (has been!) a long work-in-progress, but here's my input:
1) Might want to see if you can use the old UBB code for HTML generation and port it over to the new PHP Version. I'm on dialup for a while, and I remember pages loading quite a bit faster in the older Version. I'm using Firefox right now, I'll be testing in IE in a bit, but there's probably a few streamlining options that are lurking around in the old code.
2) Stylesheets in the profiles are a must, even if it's just a few static ones for font sizes. Decreasing the font size in Firefox makes things more managable, but then it throws off fonts for all other sites if it's made permanent. A suggestion from a webdeveloper standpoint; use pixels for font size in the stylesheets rather than points. Different platforms implement point sizes differently, but pixels are constant across all platforms. In a stylesheet, that would mean using something like "10px" rather than "12pt". Exact numbers depend on the particular font, but I believe 10 pixels is equivalent to 12 point for Arial on IE/Windows platforms.
3) Probably a no-brainer, but might want to run a search across all Posts for the UBB Code text for URLs (I believe it's [URL link=...), and replace it with actual links. And I would take the forum offline for a bit to do it, since it most likely will take a long time and do terrible things to your server
4) The layout in some areas needs a little bit of work. Particularly in the Reply/New Post area, since I'm looking at it right now. Everything is lined up vertically, and there's big beige space to the right of my screen with absolutely nothing on it.
5) Bring back the iframe with the thread in it! It helped a lot when replying to threads.
I'm sure I and others will think of more things later. Thanks for all your work Richard and (excuse me while I open up a new tab to check what your wonderful wife's name is since there is no more iframe for me to check it in
...) Mindi!