Add a vote for the simple "minimum post count".
I kinda like the signatures because, like the avatars, they help identifying and characterizing a poster / user, IMHO.
On some forums there are strict & specific rules about them (like heigh in pixels, number of images or smiley, etc.), but I think that we just need to enforce some "taste". When a signature show up that will be unapproriated enough, I think one user or the other will find a polite way to notify it to the author and suggest changes.