How about this (maybe you have to reload/clear cache as the CSS file has been edited)? Actually it is mainly a template modification of the former "Fold threads" feature.

1. The majority of users in my forum opted for the old mix-view; therefore I'm using it as my default.
2. I like it also myself, but
3. the table layout was problematic

I agree with bttr that the former 'fold threads' was nice (maybe we can keep both options alive).

Right now space of the viewport is 'wasted' since the two divs containg the latest postings / the tag-clouds prevent the table 'spreading' to the right. IMHO this can't be solved by means of CSS (the width of the table is chosen by the rendering engine of the browser in the first row of the table).
A possibility would be to move the recent postings / tag-cloud divs to the top of the page and modified them in a way that they consume just a little vertical space (mainly shorten the lastest posting to something like: 'Subject 1' - HH:MM ago, 'Subject 2' - HH:MM ago...)

Remark: the font-size of 'Category' field does not scale.