I was thinking the other day (I do that from time to time, you know) about why I like Captain Jack in Doctor Who the Ninth (and apparently in Torchwood! Woo hoo!). I'd say the reason why is that he is, in a manner of speaking, evil.
Y'ever watch Ally McBeal? Nice little show. In there, Fish was the same sort've evil. Maurice in Northern Exposure was also a similar sort've evil. Murphy Brown was much the same way in her own show, as was Dan Fielding in Night Court. -- Rarely do they do things for other folks out of the kindness of their heart. It's generally a matter of if things are going swell for them, they coast and let the good things happen. If things are unsolvably bad, they coast since there's nothing they can do about it. If things are partway in-between, they see what they can do to work them up to "good". If they help out other folks, it's typically so that they can be sure that they can get something out of that person in the future.
Jack's personality is the sort've thing that one typically finds in the right-hand man under an antagonist, and is (relatively) rarely found in someone who is supposed to be "one of the good guys". Or at least, such is my view on him. As is seen in his very first episode, where he saves Rose not because she's a helpless person in a horrid jam, but because she's cute and he stands a good chance of getting some sort've "recreational repayment for his kind act" (*AHEM*) if he saves her.