First Class dress could be it's own thread.
If I am going on a day-trip, or off the plane to a meeting, I'm in my business suit on-board. I HATE flying that way. If I can possibly manage to get to a hotel first, I fly in jeans or chinos and a golf shirt or flannel buttondown. I've never received poor service or dirty looks because of this. I'm neat, clean, my requests are polite and respectful, and I'm treated well.
What I can't get over is the people in FC (usually men) who have their shirts half out of their pants, change pullover sweaters in the aisle, smell like they've never seen a shower, or sit and assume that the entire row (my seat and armrests and tray area included) is THEIR desk and begin to sort tax receipts written on napkins and bottlecaps.
How do these people get into FC? There must be a lot of eccentrics out there.