Back in the day, when I was a little girl, my father always wore a suit when flying, even if we were going on vacation. Those days are long gone.
When flying on business, I usually wear a pantsuit and blouse. I used to have a blazer with many pockets, and it made getting through security a breeze - I kept my wallet, keys, and cell in the pockets, and when I got to security, I just threw the jacket in a tray, put my bags on the conveyor belt, and waltzed through the metal detector. The blazer is long gone, but I still do the pantsuit and blouse with trouser socks and flats (comfy, and easy to get out of and back into at security).
When flying out for a vacation, it's jeans or yoga pants, tank top, jacket, trouser socks, and flats or sandals. Most of my wardrobe is Chico's Traveler's line, which is acetate and a bit of spandex and very comfy.
If you want to wear a t-shirt, that's fine, but think about what's printed on it. If you're going to a social gathering with people you haven't met before, would you wear a t-shirt with an expletive or offensive comment on it? Probably not. So don't wear it to the airport.