I think the key is the difference between what is comfortable, and what is scruffy.

If I have to get on a long haul flight in a suit (say if I am going directly from office to airport) I will change either in the lounge or on board. If I can dress, then I dress casually, but neatly.

I do not tend to wear shorts or short sleeved shirts, because I find that my legs and arms can get cold, especially on long flights. I will usually wear khakis and a button down shirt.

I always wear laceup shoes (to deal with swelling on long flights) and I always bring two pairs of socks.