When I was watching this movie and heard that Kennedy had been "one of them" I of course immediately wondered what his mutant power could be. And just as immediately I heard someone deep in a trekbbs thread about this movie say "his dick". And then I started laughing into my popcorn but I'm sad that no one has said such a thing yet because it means I don't have prescient powers. Or maybe they only apply to ME which uhh.. is quite useless.
If it's any consolation, I jokingly had the same thought when Magneto mentioned JFK was a mutant.
If it's any more of a consolation, so did some of the people making the movie:
“The notion was that Trask was a little bit involved in the assassination of JFK. I’m just telling you back story, it’s not in the movie, but Trask identified that JFK was a mutant, and then the people that wanted JFK killed for all their political reasons, whoever it actually was who actually killed him, they were motivated by the fact that this guy was a mutant. So Eric got wind of that, and tried to stop them, but there were too many of them.
“We talked about what JFK’s power was for quite a bit, and my answer for you may be a bit obvious and not that smart, but it’s a telepathy, a mind-control, the ability to persuade people to do things. There were other theories from other people that were slightly more… tawdry.”
http://www.empireonline.com/features/days-of-future-past-secrets-explained/p13