Everypony save for Celestia will be very awkward, overly formal and probobly a bit scared of Luna, I'd imagine.
As said before, nobody really knows her, they are too used to just Celestia and before episode twos end, she (sort of) was a villain.
Though we still have not gotten a canon explanation how Luna became Nightmare. All we have to go with is that Luna grew jealous because of ponies shunning the night. That's all the show has actually given us. No mention of mind control or anything.
And no, not even Discord did it, Luna helped petrify him before her shared rule with Celestia. It was much longer after that she became Nightmare.
As far as anyone can tell, Luna just
snapped and went on a rampage. So it's perfectly justified that others would be scared of her a bit, even if she's nice now, they wouldn't want to upset her out of fear.
And Luna seemed very sorry at the end of episode two, so of course she regrets the whole thing.
EDIT: moved below...
edited 3rd Oct '11 5:39:08 PM by KuroFox