My question about what makes firstt class was intended more to stimulate discussion than as a serious request for information, but a lurker has PMd an explanation of the official bus classes:
"1st class: luxe, with AC, if run by private contractors often labelled 'VIPยด; should have an onboard toilet (which does not guarantee it's usable!). Mostly non stop or at least no official stops en route.
2nd: basic AC, just a seat and nothing more. Nowadays mostly minivans; will also do intermediate stops
3rd: rarely seen today, old style orange non-AC does all stops, takes each+everything onboard etc. Also the big songthaews that serve most very local provincial lines.
4th: double deck bus, with AC, very few of these, intermediate between 1st/2nd. Prone to accidents and thus mistrusted by most Thai now."