Originally the character of Draco Malfoy was an annoying wimpish bully, and for that aspect Tom Felton did a pretty good job. Nobody better to play an immature kid than an immature kid. Over the next four films we got this idea in our heads that he was going to be one of the main villains by the end of the series. However, in Half-Blood Prince, Felton got a chance to fully round out his character as somebody who has been raised to be a certain way and wants to be that way, but he just isn't.
Felton had been forced to play such a douchebag for so long that we didn't really think of him as a very good actor. It's only when you add humanity to a role that you can tell how great an acting talent is. In a film where every character has a sad story arc, Felton had me reaching for my metaphorical tissue box, which they wouldn't let me bring into the theatre for security reasons (I don't know why), quite a few times. I'd like to see him be one of the young actors who goes on to take bigger and better roles after Potter closes up.