In English, politeness is pretty straightforward - you say please, thank you, excuse me, and you pretty much have it covered. Someone says hello, you say hello back. If someone gives you a compliment, you say thank you.
Not so in Arabic.
In Arabic, when someone greets you, your reply must always be MORE polite than the greeting you recieved. When someone says "good morning" to you in Arabic, you reply "sunny morning" or "flowery morning" (or Egyptians might reply "honey morning"). If someone says hello (marhaba), you reply marhabtain ("two hellos") or if you really want to lay it on, "meet marhaba" (one hundred hellos). If you give someone a compliment, for example on their clothing or the cuteness of a baby picture, you must always follow it with "Mashallah" - God wills it - to show you have no envy and ward off the evil eye. After you share a meal and someone wishes you "sahtain" - that it brings you health - you reply "a'ala albik" - "health for your heart, too".
They are all very beautiful phrases, I just wish it was easier to be as polite as I would like to be. It would be nice to just say thank you (shukran) and have that be it - khallas! But since it's not, I think I have some studying to do....