If there's one thing that stands out to me in a great book or television show, it is specific characters. In my mind, these personalities (or lack thereof...) can make or break a show. You can have awesome plot twists and terrific writing, but if you don't have characters that people connect with, it things can fall flat.
We like variety, diverse and dynamic characters. As I've said before, we love personalities that jump off the page because they seem realistic and charismatic. They're the people you want to learn more about with every chapter or, in the case of a television show, each episode.
Characters should grow, especially in a series. Just like everyday people, it seems a bit unusual to not see growth or, at the very least, a change in opinion over time. As you learn about the characters, they're often learning about themselves. Even more interestingly, the characters you're drawn to might also be telling you something about yourself.