Like most things regarding the leadership feat, it's mostly up to your DM. There aren't any rules for Great Renown or Fairness, either.
The way I run it is that Special Power means that you have abilities that are both public and not typically accessible by other people. For example, if a warrior is known for using a sword that cancels all spells on people he hits, that would be special power. Likewise, if a wizard had a reputation for coming up with new and powerful spells that other people couldn't cast, that would also count.
Basically, if the cohort or follower you're trying to attract knows that you can do something that other people can't, you get Special Power. But again, this is the kind of thing that's mostly up to your DM.