I think the best way to describe roleplaying is as acting - thats all you are really doing, playing a roll. So, you're not some guy or gal sat on their computer playing the game, you are the dwarf .. so you talk like one, act like one etc. whenever you are meeting someone in the game.
Some of the best examples I can think of
- an elf who could not speak 'common' very well, she used to used very broken english when talking and use some Kaldorei words
- an forsaken warrior who still lived by code of honour he had in life as a knight.
- the young gnome who is trying to find his lost uncle who he has not seen since gnomeregan fell.
- a guild of allied magicians who had their base at the tower of azora in Elwyn forest, researching magics.