The 'How you doing?' line of Joey in Friends is very famous. English is my third language. I seem it's general conversation dialogue. What's so special about it?
The expression How you doin'? is a modification of the English How are you doing? which is another way of saying "How are you?".
This phrase seems to be used by stereotypical mobsters in popular culture, who usually were of foreign descent and so would pronounce expressions with an accent. For example, Tony Soprano in the third season (the 5th quote in Another Toothpick):
Tony Soprano: How you doin'? I got new shoes. Soles are a little heavy
Joey does the same (probably a reference to his foreign descent) but in his case, the sentence is a pick-up line, a conversation starter for his die hard promiscuity with women.
As others have pointed out, the phrase in itself is very innocuous and mundane, but it's mostly the WAY he says it that adds the flirting overtones. Notice how when he usually says it he narrows his eyes and raises his chin in what he must believe is a seductive look.
I also think it says 2 things about his character:
- He is not smart enough to think up anything more original
- And/or he believes he is so good looking he doesn't have to say anything more to attract women