First, Business, or Economy, Domestic or International, I usually wear a loose (one size too big) dress shirt with no tie, slacks, and deck shoes. On occasion, I will swap the dress shirt for a polo.

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.

I tend to fly in slacks and a polo or my black standard-fit levy's but sometimes I go in my slacks, dress shirt, or slacks, dress shirt and tie. If I'm going somewhere cold, then also a jacket, otherwise I pack the jacket if I'm bringing one at all.

Men in sandals = Indecent Exposure IMO.

Wow, yeah; that would be a situation I would hate to run into but certainly; I see a lot of women in thong shoes and I'm like, "those wont hold very well if you gotta go RUNNING out of the airplane in a hurry, heh."

Just so long as the passenger IS wearing something, I suppose.
Hey, I wouldn't mind at all if Milla Jovovich wanted to fly next to me in any cabin in just a thong swimsuit.

I guess it's one of my pet peeves of people who dress really slovenly (in the old days people used to dress up for travelling on the plane) these days.

It's OK to dress casual as long as it's neat, clean and you are groomed properly. I avoid shorts and sandals because of the A/C. I try to wear deeply patterned shirts or at least avoid white shirts knowing that I tend to spill things on my shirt whilst inflight.

I also agree that if you are dressed up a bit, and act in a professional manner, staff tend to treat you better, which may come in handy when you need their help (e.g. when flights are delayed, canceled, etc.).

I always dress for comfort and practicality. For long-haul flights, I find jeans too heavy and restricting to sleep, so generally wear a smart/casual pair of trousers like Dockers which I find much more comfortable than jeans.

I always wear a micro-fibre shirt that does not need ironing, and prefer to have a pocket in the shirt to keep my passport and boarding pass while walking through the airport so I don't have to keep digging into my carry-on bag. On BA flights, I always wear a short-sleeve shirt since their cabins are always a few degrees warmer than I would prefer. Otherwise the sleeve length suits the destination weather conditions, and since I often change hemispheres, that often means changing season between source and destination.

I find my most confortable shoes for both walking and just wearing for long periods.

On long-haul flights I always carry two spare pairs of socks, two spare shirts and two spare pairs of underwear, and will grab a shower at every opportunity I can at intermediate airport lounges.

Cultural differences I guess. To me it looks perfectly normal (as long as they don't include socks).

Several years ago my wife and I flew to Europe from the U.S. on USAir buddy passes. Our friends had read the rules and had told us they said we needed to be dressed in something better than jeans and sneakers if we wanted to be able to buy an upgrade for $50. I thought that sounded like hogwash (I didn't understand at the time what a buddy pass was), so we showed up in jeans and sneakers. No dice, while our friends, also on buddy passes, got business class seats for $50. On the return flight, I put on my one pair of khakis and the hiking boots that were the only other pair of shoes I had brought. We almost got the $50 upgrade, until the head check-in agent saw my shoes and said, "No way am I letting you upgrade in shoes like that" (and they weren't even muddy or scuffed up or anything).

These days there's so much less left to the discretion of gate agents or flight attendants, that essentially all of my upgrades are earned. But I still dress reasonably well because it can make a difference when dealing with someone who does have discretion, or just in the way I'm treated.

I'm usually changing hemispheres when I travel so I have to be aware of my destination.

I usually wear casual button-up shirt with pocket, dockers or jeans, light jacket, lace-up sneakers. I'm considering shorts in future because on flights where the air-cond has an impact there is usually a blanket included. It's rare for me to sleep on the plane so I don't have to worry about that. With spring/autumn flights I only have to worry about the chill during an early arrival time. When I arrive in a warm place I carry my jacket on top of my bag.

I usually take my shoes off in flight; I don't like wearing shoes and on the plane I don't expect to be walking on rocks or glass. I swap my socks for the supplied airline socks in place; no sense in in wearing mine out. If the bathroom has a wet floor from other travellers I'll either use another or I'll return to my seat for my shoes. The wet floor will be from the basin anyway. (As I don't sleep I don't have to queue with the other travellers to use the bathroom.) One flights of less than an hour I'll probably leave my shoes on.

Yeh, really. Stay out of 40% of the world if you dont like them that much because in Asia, North Africa, and the Middle East it's about all you find a lot of times. Generally anywhere that's hotter than hell. But you also find them in South America, WA Australia, and the SW United States. Personally, I find that being cooler is more important than looking cooler.

If I'm not going directly to the office, I usually opt for jeans, tennis shoes (no metal), and a shirt with a pocket for my BP and/or passport.

The shoes are for comfort and (hopefully) avoiding shoe carnivals. The jeans can have anything spill on them or stain them and it doesn't bother me (too many filthy cabs).

I guess I also heed the warnings about folks who have been in emergency egress situations. I try to wear natural fibers and sturdy shoes. Not always long sleeves, because I think I've found all the planes where the a/c isn't cranked down to "Meat Locker".

I've done the suit and tie or business casual thing, but it seems like the clothes end up looking like I just sat in them for three hours (which I just did, heh).

I guess if somebody is trying to test a theory about class differences, I don't think there is a pattern. The suits tend to be salespeople, by and large, or more senior execs who can't get the corporate jet (or whose companies don't have them).

Stretchy pants (think Gap Stretch khakis) for comfort and lack of circulation issues, tanktop (again comfy), and long sleeved button up shirt (I really dislike the sensation of someone passing by me on the plane and brushing up against my bare skin. This applys to people in my row who insist on taking over the shared armrests and pressing up against my arm.) Shoes are normally close toed because I tend to get cold on planes.
I basically try to look neat and somewhat dressed up since I'm young and I found that dressing nicely helped get me better service than the times I looked liked a grungy college student.

Well, if you ever run in to me while traveling (and I hope you do), I'm always dressed in Nike. From the shoes (no metal) to the pants, tops and jumper or top. It's usually a track suit because with the new Dri-Fit stuff (fabric that breathes), it's absolutely wonderful on long hauls and i can easily rinse and wash to dry over the hotel room vent for the trip back. I doubt if I've worn anything but this for the past 18 years traveling. If I've got biz on the other end, I slip into the stalls and quick change myself into the zoot suit, same with arriving at the airport after some biz. I don't think i've ever fown in a suit.

Of course the question i usually get from someone picking me up or meeting me is have i just come in from taking a run or from excercising. I have found sportwear to be the most comfortable. It's always black or blue so that the dings and spills aren't noticeable, especially when i've been attached by a child sitting in front or behind.