Maybe the common enemy IS this system that America rests on. Maybe the common enemy is the fact that the same diseases affect everyone...whether they be AIDS or a lack of responsibility...they're all problems that affect everyone in one way or another. Maybe the common enemy is the people's lack of vision that keeps them from realizing that all of this politics bullshit won't last and isn't real. Maybe the common enemy is the fact that we as humans are decendants of warriors and hunters...all around self-suficient people that bought into convenience long ago and have since taken a downward spiral to what we are now. I could go on with these, but the fact is, there's plenty other than ourselves to fight.
Also, did you ever think that maybe the fact that there is no set common enemy is the very reason everyone should unite? If there's nothing to fight, there's no reason for the teams we have divided everyone into.
I agree that many people have turned it into empty rhetoric that's fun to say, but the basic idea that it started out as has a lot more merit than you give it.