^^
Well, let's face it, you've got the cream of British heavyweight acting thesps all through these films - hardly surprising kids who seemed to fit the bill when they were much younger haven't turned out to be amazing after all.
I did think Tom Felton (Draco) made a fair show of inner torment as he grapples with the conflicting emotions of doing what he believes is right, fear for his family and the pressure to step over the line from privileged bully to murderer.
As for humor, given how British (and based on boarding school cliches) it all is, even tweaked to be a little more international, I'm amazed it travels as well as it seems to.