It's how much evil can the boards handle before you get banhammered.
It's the above idea that makes a good villainous game so hard to find. When most people see a villainous game, they run to make the most evil, bad ass, serial killing murderhappy idiot around. Evil doesn't see itself as evil. People don't just wake up and decide to do evil, they do evil for reasons.
A memorable villain, a memorable antagonist is one whom you, the reader, can sympathize with, or at least empathize with his motives. An excellent example is Magneto, he is protecting his people from those he views as their inferior, but those inferiors outnumber his people by so much he goes to extremes to defend them. Doctor Doom, Black Adam, Darth Vader, characters like that stand out because you take a noble emotion and crank it over 9000, and it becomes twisted ... BUT
... it started off as a noble idea, good intentions.
It's those characters who are memorable, not the psycho killers, or the lone wolves, or the ancient evil given human shape out to enslave humanity. That's just bad writing.
Did you have a PL or PP total in mind for this? I ask because I would suggest giving more points then the usual 15/PL. If your making a villain, you want to be able to be big and flashy. Its a chance to really get out there and let lose. That said, it also helps because unlike hero's, villains really need to be much more self sufficient.
The above is absolutely not true, a villain in a team of villains needs no reason to be more flashy than their nemesis the hero, in fact I feel a villainous game you would need to have your characters more reliant on one another to prevent them from getting at each others throats, if everyone is self sufficient, why work together at all?
Again, my opinion, and just a general comment on why I haven't seen a villainous campaign ever really work ... /shrug