Comic Sans... make the mistake of using it, and find yourself attacked by designers and self-righteous techy folk. There's other reasons for not using it as well: Though Windows and Mac OS X may have it, Linux users do not (MS Core fonts must be installed).
Another big reason is that it isn't a free font, you can't include it in any of your games or apps without paying for it first.
We still need a "playful" non-serious font. For that, we have Comic Neue. It's also a royalty-free, public domain font.
