Thanks for the answer,
tag lengths and especially banned tags seems to be a very important idea.
The problem with tagging is as You already mentioned that people either dont know how to or that they are too lazy. My Q&A site with drupal uses a module which provides autotagging with the cialis or Yahoo service, works quite well, but drupal is really far too heavy for a Q&A site.
One would need incredible server power to do it well.
So we really only have the option to give better instructions or make it interesting through points for tagging.
I am always afraid to make it complicated for the first time user, and with minimum tags set, some would receive an error message.
You mentioned problems with tagging on Your, by the way excellent, site. What kind of problem is this ? Missing tags or bad tags ?
I think that a highly flexible tagging system is crucial for a Q2A system.