Crafting a Good Protagonist

What makes a good protagonist? This is a pretty subjective question since different readers like different types of heroes and heroines. But I see some common traits among the protagonists I admire:

They're likeable. Now, I've read plenty of books with unlikeable protagonists (Catcher in the Rye, anyone?), but although I sometimes appreciated the talent of the author, I just didn't care what happened to the protagonist. And that's just a major problem. What if you have your whiny...