IMO the main problem is a lack of interoperability and valid alternate implementations. The syntax is currently literally defined as whatever falls out of the PHP parser. That's not good enough for the big world and people are routinely horrified when I explain that's the reason MediaWiki doesn't have all manner of obvious useful things like a WYSIWYG editor, format converters, etc.
Agreed. But I would also add that a nice (but low priority) characteristic of an alternate parser would be that if can be easily reused in other apps besides MediaWiki. Lots of web apps seems to reinvent some HTML-lite syntax, and it would be beneficial if multiple apps could just drop-in our parser (thus saving users for learning yet another syntax, and saving programmers from having to reinvent the wheel). Having experimented once a long long time ago with extracting the parser, and having found that it required lots of setup and global variables to work, I can understand why this doesn't happen currently. So a loosely-coupled parser that encouraged reuse could be nice, and is related somewhat to encouraging interoperability.
-- All the best, Nick.