What a great topic! I have so much to comment!
First of all, there is a huge difference between articles that can be labeled under the same category. Here are a few examples.
Jeans: While I very rarely fly in jeans that are as light coloured as Fallap's in the photo, I think that some jeans are very appropriate to wear on board. I tend to use dark blue, dark grey and black jeans, which in my opinion can look very stylish if combined with the right shirts, jackets and shoes. Another thing is that I in general tend to use more tight/slim pants, which also in my opinion looks much better and neat than very lose clothing. In my opinion, wearing a suit, unless you are going to a meeting, is exagerrating.
Sweatpants: When I was in my early 20s, I sometimes flew longhaul in sports pants (not sweat pants, thinner and different material). I hate myself for doing that! But even then, I chose black sports pants that would match my black/dark non-sports jacket. Nowadays I always enter an airport and a plane in long non-sporting pants, but on long, over 6h flights change into sweatpants after take-off and back out of them prior to landing. I love my slim Abercrombie sweatpants, which look very nice and feel amazing! If you've ever worked out in the sweat pats you're wearing, you shouldn't wear them when flying, even if they are clean.
Shorts, flipflops, sleeveless t-shirts: I hate it when people are flying wearing any of these! It looks awful on anyone! Seriously, has anyone ever been warm on an airplane that is not standing on the ground? It's always freezing, no matter where you fly! Never too hot for thin long pants, a normal t-shirt and some damn socks (get thin short socks)! Even most airports are cold. I'm very proud to say that I've never flown wearing flipflops or sleeveless t-shirts and hopefully never will. Once I flew in shorts, because I was going to
SXM from
SJU and it felt very appropriate

Even then, the shorts were one of those that you might wear in a city in the summer if it's really hot and not those that you wear on the beach.
No matter what you wear, what really matters is what's underneath your outfit. I always take a shower either on the morning of a flight or the previous evening if flying out early. And I always have a deo stick with me to refresh the armpits either on a layover or during a flight. I hate it when people smell, no matter where the smell comes from - armpits, feet, mouth, general smell. That is one of he reasons why I hate people wearing shorts, flipflops and sleeveless t-shirts. Their smell is way too close to me and way too exposed, cover it up!
Since Fallap shared his photo with us, I think it's ok to write a few comments. Overall, the outfit is neat, but not really stylish. If you sat next to me on a plane, I would be happy, since things could be also much worse. Your photo is dated March 2013, so I assume you are flying to or from
CDG. I would not wear pants that light/pale in Europe at that time of year, only in the summer. No matter where I was flying. I agree with AyostoLeon that your outfit can't really be regarded as "dressing up", but unfortunately for the wast majority of young people these days that is the case (not saying that you are necesseraly one of them, Fallap).
Fallap asked us whether we dress up or dress down when flying. I wouldn't say that I dress up, since I always try to look neat and stylish when leaving home. Less of course when going to the supermarket than when going to the city center. But even if I wouldn't call my outfits "dressing up", I always try to make it a little bit special when flying. After all, no matter how many people get to fly these days and how many of them try to make flying soud very "everyday", it is special, no matter if you fly twice a year, about 40 times a year (like me) or hundreds of times a year. It's also what you make out of it. If you dress up, behave a little bit better and polite than usual, the experience becomes much more pleasant than if it's just another way to move from A to B.
[Edited 2013-12-08 03:41:08]