I understand it's acceptable to wear something comfortable during a longhaul trip (track suit, leisure suit or alternate '70s attire if you must) as long as you are dressed in what my granny would call appropriate clothing at the beginning and end of the flight.

However...if you choose to board wearing a retro loose-fitting Adidas track suit with too many gold chains and a flat-brimmed ball cap turned askew, you may be presumed some sort of celebrity and thus given all the deference (or disdain) to which they are accustomed. You'll probably be 'outed' for flying in J rather than F, but if you follow g46r's advice, most people will believe you are indeed a star of some unfortunate reality show after all.