Ruby is slow and not as widely supported as other langauges, MySQL is more unnecessary overhead, and there's not much to gain from using AJAX, but you'll be shutting out all kinds of users unless you supply proper fallbacks. Neither 0ch, shiichan nor kareha use a database at all, because there is no need for it.
The problem is that you're basing your design on what tools you want to use, not the other way around. That's no way to make good software.