underscores in URLs: e316af145b6c really wasn't the right solution (although technically fine), and it breaks on a bunch of sites that don't handle percent encoding well (notably GitHub)
Don't allow more backticks directly before or after the code span opener. a) The "spec" strongly implies that this is the intended behavior, and b) it prevents a backtracking catastrophe with long strings of backticks.
change meta.stackoverflow links to meta.stackexchange
optionally (since it's a pretty big change) handle non-ASCII letters like their a-z counterparts when doing the bold/italic step