I used to think there were major differences, but now that I've flown so many times on so many different airlines I feel that for the most part they are the same.
However I do think there are always some differences worth mentioning. If one is flying from DFW to just IAH/HOU or AUS, then there will not be much difference between WN, AA, DL or CO. But if one is flying from LAX to ORD, they may get on an AA MD-80, no movie, no audio and maybe a deli bag. Or they may get a nicer plane. If they are on United they will be guaranteed an AIrbus or better with full audio, video and a nice hot meal. So it depends on the routes, and the competitors. On the long haul, some are better than others, but it is determined by a customers prefereances, i.e. most people may not care if they are on an MD-80 and there is no movie, I would.