After collecting some feedback and following some of the threads that have been started up, I've decided to do a second thought about my first proposal.
It had too much uncovered use cases, and still it didn't solved some major problems from the current one. To save some time and try to focus on the UI design issue, I decided to just do an inkscape mockup instead of a working solution.
This proposal tries to solve a few problems, and it's somehow inspired by gnome-specimen from uws. First, it gives an overview on how each style would look like without the need to click on each of them, and it grants more space for the preview. It also adds a way to search for certain typeface classes, monospaced, language specific using a thunderbird like search box.
Some problems that I'm not sure how to solve here is, how to properly let people edit the preview text, I think that the entry box on top is fine, but I have the gut feeling that it could have problems. Also, I'm not sure how hard to implement the search box would be.
Anyway, feedback, again, is welcome.