For shirts, I'll pick from the ones the REI sells that look nice enough to be business casual, lots of colors & styles to chose from, comfortable enough to wear on long flights, and durable enough for active pursuits at my destination...think "versatile." AND, when I buy them thru an airline mall's website, I'll get miles for them! (has come in handy when NWA does their Fly-Free-Faster promo's over the summer).
I do see more and more people wearing shorts on planes now, incl. cargo shorts, or those North Face zip-off-pants shorts....
I do remember times where I've "flown south" during the winter, yet had to sport the full winter gear incl. jeans, to shovel a lot of new snow & ice off my driveway, to get my car out in time to make barely make my flight---and looking like some kinda' winter-wonderland dork when connecting at...MIA...in December. Couldn't just leave the gloves & hat & sweater in the car, since they're needed when returning to the cold. And, At 6'6", I just couldn't maneuver changing in the MD-80 lav. en-route to MIA.