Wickham's good points are that, because he was raised at Pemberley, he has just as much of a gentlemanly air as Darcy. He is also very handsome, tall, and very polite. He converses very easily and lacks arrogance. Elizabeth was easily beguiled by him because he was so friendly and easy to talk to, unlike Darcy, who is reserved and has a sense of pride that makes him seem arrogant.
Wickham's negative traits are that he is manipulative and a liar. At supper at the Philipses, Wickham engages Elizabeth in conversation because she is attractive and he suspects she is rich. Probably having seen her take note of how Wickham and Darcy received each other when they met on the street in Meryton, he begins a conversation about Darcy. He feeds her plenty of lies about how he was mistreated by Darcy, when the truth was that he had tried to take advantage of Darcy's sister.
Another negative trait is that Wickham is mercenary. Everything he does, he does to try and capture a rich wife for himself. He flirts with Elizabeth when he first thinks she is rich, then switches to Miss King when she inherits £10,000 from her grandfather, he attempted to seduce Miss Darcy for her fortune, and even though he let Lydia run off with him, he had no intention of marrying her because he had still hoped to find a wealthy wife. He is bribed to marry Lydia by Darcy.