When you are on an airplane and you have an aisle seat, it's your responsibility to keep your elbows, knees, shoulders, toes, arms, feet, head, blanket, backpack, and anything else you brought on board OUT. OF. THE. AISLE.
"But why? I paid for this seat so I would have extra room. Have you seen how small these airplane seats are?"
Dude. If you want extra legroom you'll have to spring for an exit seat or business class. And yes, believe it or not I'm actually quite familiar with airplane seats. I only spend about a hundred hours a month on airplanes, but thanks for the update.
So why is it so important to keep the aisles clear?
1. Safety. When there's an emergency on a plane, your flight attendant needs to get to the situation as quickly as humanly possible to assist the passenger in distress. If you're blocking the aisle, you're endangering the safety of your fellow passengers AND the safety of your flight crew. I had a one flight where a passenger became very ill and projectile vomited all over her boyfriend sitting next to her (he handled it flawlessly. I was so impressed.) and when the call bells around her started dinging, I booked it down that aisle, tripping over the feet of oblivious passengers all the way. Frankly, it's a miracle I didn't fall and add to the medical emergency we were already dealing with. 
2. Speaking of injuries, did you know those beverage carts weigh 200-300 pounds? When they bump into your elbow or roll over your toes, it's going to smart. A lot. I do my best to warn you we're coming through, but if you have headphones in or you're asleep and you have limbs dangling in the aisle you could very well end up quite hurt if you don't move. Be careful. I'm trying to keep you safe. Work with me here. 
3. The aisle and the galleys on a plane are our workspace. How would you like it if I came to your office and sat on your desk, wouldn't stop talking to you during your lunch break, or blocked the path to and from your cubicle? Oh, you wouldn't like that? Ok, then please don't do it in my office space. If you desperately need a cup of ginger ale because your stomach is upset, that's fine. If you need some water, that's fine too. Come on back and see us! Then kindly leave so that we can continue to do our jobs.
For my safety, for your safety, and just because it's polite- be a responsible passenger and please keep the aisles clear. Just do it.
All of these comics are from the brilliant JetlaggedComic. I'm obsessed. http://jetlaggedcomic.com
