I like the fluid forum view for staying on top of the forum contents, but the categories list takes up a lot of valuable real estate on a page we are trying to make the most of. I suggest making the categories box collapsed by default, and then allow the user to expand it when desired. Maybe this can be done in a drop-down box like the compact view does it.

We have a side bar filled with other stuff, and its a little awkward that the fluid and traditional views result in a side bar with a much different height.

Another solution could be an option to just hide the categories widget altogether. I have fluid and traditional views enabled, and I switch back and forth based on what I am doing. If I want to drill down into categories, it's easier just to switch to traditional view.