I think 4 lines including the name and any links is about right, hopeully that's what'll make it into the guidelines or something. I think one Muppet quote is ok, but i don't see the need for like 2 or 3, it's gets annoying seeing them EVERY time someone posts especially as most people here are hardcore fans anyway. It'd be nice if people rotated things in their sigs every week or two to make it fresh.
I definitely agree with Jamie that people shrugging and having a 'who cares?' attitude is not needed around here, the forums for everybody, not just one individual. If you are posting something that is shorter than your sig it would be the courteous thing just to untick the 'show signature' and leave it out for that post !