All hail the fascist space-oligarchs! Wow, the future's looking bleak, isn't it? Unless we acheive trancendence and leave our mortal forms, pretty much every avenue looks like history is either repeating itself or taking the piss.
Well, I mean, I doubt we're gonna become this peaceful utopia when we spread to the stars. If we ever colonize out there, it'll probably be like MechWarrior or Dune.
I think a major, often overlooked problem would be microscopic pathogens, and the fact that organisms on either side would not likely have any resistance to such pathogens a different planet or moon.
Like everyone else has said, our pathogens have evolved to attack us. They wouldn't do much to an organism that doesn't have DNA or similar proteins/body systems. Same goes for alien pathogens.