The problem with the actors in the PT is that they weren't given anything at all.
The main job of a director is directing the actors. The term "Director" comes from this. All Lucas did in the PT was oversee the special effects, the CGI, the models. You can see all this in the truly excellent extras on the Phantom Menace DVD#2.
Lucas even says that "we hired good actors and we let them do what they do best. I'm not worried" or something so very similar.
If Lucas would've put as much effort in the direction and depth of his characters(who are full of potential, even Jar Jar) as he did for his little toys, the PT would've been fantastic.
But in the end, the PT is a six hour toy commercial, nothing more.
Oh and by the way, George Lucas wrote, produced and directed A New Hope.