Mostly it's the tailgaters that I don't like.  It's not that I'm not going the speed limit.  Usually I'm going as fast as I can without getting unwanted attention from the highway patrol.  It's not that the tailgater cannot get around me.  We could be the only two vehicles for miles in both directions, but some people will still try to ride the rear bumper.  They need to either pass me or back off. 

I was once the passenger of a person who did this to other vehicles.  I watched in amazement as this otherwise rational person seemed to think nothing of riding other peoples tailgates.  It seemed as though following at a close distance was simply in his comfort zone.  I don't think the possibility of the person in front of him getting a severe case of road rage ever crossed his mind.

Edit: I don't mind the old folks as long as they are capable of following the rules.  If they cannot drive for themselves, someone else will have to do it for them.  (I've been that person a lot more often than was convienent.)  People cut me a little slack with the big equipment.  Likewise, I can put up with Aunt Betty slowly accelerating to 54 mph and driving ten miles to meet her independent living needs.