I normally use VBulleting forum, several of my sites runs it but I algo use wordpress in some sites and for a new one I was looking for something in the middle betweeen forum and blog and I thing I got it with bbpress.
I know it isnt like WordPress but I like it.
To the point, in VBulletin I got a automatic thread tagger, it will add tags automatically to the posts, it works just fine, you can exclude words like TO, FOR, THE, etc.
I guess something like that is missing here. I dont know how to code it.