Actually, // and ** are at least as clear, and are most definitely parsable by a fixed-lookahead context-free grammar - even an unaugmented LL(k) grammar could probably handle it. <i> and <b> are unambiguous, but ugly and language-dependent. MediaWiki's current behavior "fixes" many of the issues with its ambiguous bold/italics representation with little ad-hoc DWIM-type behavior. It works, but cannot be represented by a CFG and is difficult to extend.
Another benefit of changing the double single quotes and triple single quotes to // and ** respectively is that it would be a small step to making MediaWiki markup more Creole-compatible (www.wikicreole.org). Also, it seems like a conversion script for just these two elements would not be that difficult to write. What could be potential complications?
Chuck