Princess Luna is, well, she's a regal sort and seems to be of a more serious bend than Princess Celestia. I can't say I know either of them well enough to point out anything apart from the physical, though. Princess Luna has dark fur and a mane that looks rather like she's gotten a few stars caught in it.

As for all the rest, I look forward to the future if we can get home. I have a two-seater, but flying? Having a taste of it here, I should very much like to have the chance when I'm a human again! And I'll be a jolly fellow all around. I really think you're looking too much into this business of most ponies being the jolly sort. Equestria seems a lovely place. What would they have to be unhappy about?