I'm voting for sigs and PMs. I think its absolutely insane to live without them because i hate email. I like PMs because they... work... and are convenient. I check my email almost once every 1/2 year at some points!!!
And sigs, though i don't really use them here, they ARE quite a part of forums. Sigs aren't always a waste of page space either... Some are compact. Like little bars.
But i guess it doesn't matter - i know when i develop something, nobodies going to stop me from putting my opinions going into it... Thats why, i guess, a lot of free programs have interfaces normal people would go insane trying to use
(But those same programs are my favs XD)
I might also note that people probably won't be attracted to using your forum software if they support sigs and PMs (Obviously, a lot of people do, including me... I'd go insane on mailing lists! i freaking hate E-mail. Oh, right there ARE sigs on mailing lists, the mailing software adds them *Hint, hint* XD lol)
Anyways, thats just my opinion on all this... I was thinking about using this software on my next website till now. Oh, and yes, my next website is a loooooong ways away.