I wonder if we shouldn't contact the developers of other open source wiki software, and convene a by-email "mini summit meeting" about wiki syntax, and try to formalize a standard that they and we can all follow.
There have, as always, been several attempts to standardize on some common syntax. In the Python world, reStructured text seems to be quite popular:
http://www-106.ibm.com/developerworks/library/x-matters24/
One of us has proposed a standardized Wiki-syntax ("Wikitax"):
http://meta.wikipedia.org/wiki/Wikitax
And I bet there are many other wikis out there who are thinking about the problem.
However, please keep in mind that we have hundreds of megabytes of existing wikitext, and converting this to anything else would be a major PITA. It may be doable but is probably nothing the developers want to spend much time on.
On the other hand, UseMod-style wikitext is very common. Wikipedia is by far the largest wiki and will remain so, probably forever. The best approach may be to fix the problems in our current parser and syntax, and then document that as much as possible and publish it under some name. Before we do that, we should definitely add basic support for tables, though. And \ for single linebreaks (instead of <br>) might be nice, too.
Regards,
Erik