Just a small point, but these days finding any flight--short haul, long haul or anyplace in between--that has a load factor of 50% or less is pretty much out of the question. As recently as three or four years ago it could be done fairly reliably midweek, but with schedule cutbacks, mergers and right-sizing of aircraft to save money, it's unusual to find a flight that's only half full.
I must also say that once in a while it's not so much the person next to you but the entire ambiance on the plane. A few years ago when we went to Egypt and Jordan, we found the long-haul flights on EgyptAir from Kennedy to be absolutely disgusting. Perfectly good 777s, but wow--lots of kids, lots of dirty diapers, lots of dirty diapers stuffed into seat pockets, an overall disregard for fellow passengers, aisles full of magazines and newspapers, kids running rampant with no parental concern at all, and no hope of redress because it was happening all over the plane. It was the norm.
Once we landed in Egypt, everything was fine (back before the troubles), but on the plane it was a different world, both going and coming home.