On the last flight I took, from Newark to London, I wore a sport jacket, black jeans that don't look like jeans, dress shoes, and I believe I was wearing a fitted, plain t-shirt. Nice watch on my wrist. It strikes the right balance of comfort and presentability because I can take off the jacket when I'm in my seat.
I don't mind people wearing jeans and t-shirts on planes if they're well put together, but sweatpants, workout wear, pajamas, flip flops or anything similar I judge very harshly (silently, in my mind). Like Karl Lagerfeld once said: "Sweatpants are a sign of defeat. You lost control of your life, so you bought some sweatpants"