When I registered a new project, I familiarly chose Classic, to go with Trac for wiki content, but then faced "Why do you want Classic?" question, wondered indeed, why, and choose New Forge instead. Here's one why ;-).
Just from writing one wiki page (https://sourceforge.net/p/chdistro-droid/home/Home/), I saw following issues (attached screenshots just in case):
Heading and para spacing is too big. Heading "before" and "after" spacing appear to be equal. "After" space is just enormous, heading looks detached from following text. And space between paragraphs is also very big - add single line of text at the end of section and it looks detached from everything.
Lists are dedented. It's common practice to have them indented instead, that's what look familiar and visually pleasing.
In couple of minutes trying, I couldn't find a way to render intra-wiki page link to render without surrounding brackets. The easy way to handle that would be to handle artifact links without those links at all, so "[SomePage]" in markdown source would render just as "SomePage".