Rachel came into the group as the spoiled rich girl, and while she eventually becomes self-sufficient, she never shakes the rampant materialism that defines her character. At one point, Ross suggests that she stay in the sham marriage, that they got when they were in Vegas, in exchange for wedding presents and she genuinely considers it. Greedy much?
Ross's two most defining character traits are the fact that he cheated on Rachel and the fact that he's constantly getting married and divorced, both of which point to a man who thinks who is driven by his irrational sexual urges.
Monica is driven to be perfect by the belief that she is the better at everything then everyone else, and the idea that she is anything less that perfect is foreign to her. It should also be noted that just as pride is said to be the source of the other 6 deadly sins, Monica is the central friend, that connects everyone else in the group.
Chandler made a lot of money, but it was in a job he hates. It was supposed to be a temporary after college, but he let inertia get the better of him and, it took him like 8 seasons to finally get off his ass and change careers. Could he be any lazier?
Though she's normally laid back and breezy, Phoebe has an extreme temper that comes out when she needs it. The rest of the friends seem to genuinely fear her, which they probably should. I mean, she did used to mug people, and even stabbed a cop once.
Joey eats. That's what Joey does. Enough said.