I try for the seat at the rear where the fuselage curves in. The wall allows a better prop for your pillow and you get served meals first. You're close to the can, so you don't have to walk half way up the aisle to wait. The time will come where they charge passengers like they charge for parcel rates - the heavier, the greater the cost.
Alas, they are all the longest of long haul flights - I am in Australia. It is our own fault, I suppose, for living at the edge of the world.
That is a good point about passenger weights - yes I think that they will have to factor it in, eventually. Until then, it is a mixture of compromise and co-operation.