A recent tweet led me to an interesting two-part article about "What makes a great character". This is on the blog Yummy Men, Kick Ass Chicks, a paranormal romance blog. In this article, several paranormal romance authors talk about what makes a character compelling. While I have no interest in their genre, good writing is good writing and I found the advice worthwhile. Check out part one and part two.
The Reader's Digest version: characters are interesting when they are multifaceted and flawed. Pretty much all the authors said this. They also said a whole lot more, so if you're a writer, or simply interested in how your favorite characters are made, check these posts out.