Issue: Wiki discussions have no postbit

I'm a bit confused by this statement. Posts in discussions use the standard postbit in XenForo. In vBulletin, it uses a custom postbit because vBulletin doesn't really have a generic postbit usable by different content types. We tried regular postbits in vBulletin for a while early on in the development of 4, and there were too many issues, too many openings for add-on conflicts, due to many add-ons expecting postbits only in showthread.php.

Other than that, when you open a new discussion, you have the option to make a first post or not to. If you make a first post, it uses the postbit. If you don't make a first post, you basically just create a thread title, and optionally, set the meta description. The meta description is not a post and was never intended to be. It's a plain-text short description (max: 150 characters, I believe) used by search engines and in content listings. I think the hint underneath the field is pretty clear on that.