In my family's case, elbows were banned from the table because putting them there encourages resting your head in your hand, which leads to slouching. It's not a "respectful" thing to do at the table. In actuality, it's really not hard to sit up straight and keep your elbows off the table. While I didn't quite get it as a kid, I later came to appreciate it as an adult.
As for taking the plates away in a restaurant before everyone is done, I think it's OK as long as you are asked if it's alright to take your plate. You can always decline. I have to say that when I'm out dining with friends, nobody really cares if someone's plate it taken while others are still eating. But when I'm on a date, it's a different story. I'd never let the waiter take my plate until my date was done with her meal. It just doesn't feel considerate otherwise.
As for taking the plates away in a restaurant before everyone is done, I think it's OK as long as you are asked if it's alright to take your plate. You can always decline. I have to say that when I'm out dining with friends, nobody really cares if someone's plate it taken while others are still eating. But when I'm on a date, it's a different story. I'd never let the waiter take my plate until my date was done with her meal. It just doesn't feel considerate otherwise.