I guess the best example of what Roger Ebert was talking about is Samuel T. Jackson (an experience American actor). I can think a dozens of movies where he brings interesting characters to life including the recent "Changing Lanes" and "The Caveman's Valentine". In this movie he is just a prop reciting lines to advance the plot. He was given nothing by Lucas to work with, so he looks lost in the film. I don't think those older British actors were given any more material to work with than Jackson, but their different experience and training enabled them to just "wing-it" or make it up better.
I am back to the wishing that a creative and inventive director was hired who could have filled in the blanks. I think it is no accident that the best of the Star Wars films is "Empire", where an outside director was hired and then "Return". Lucas directed the first movie, but that was at a time when he was an active working director and todays computer special effects were in their beginnings. I think "Phantom" was the first film he directed since Star Wars (a 12 year lay off). Since he originally had no plans to finish the saga, he was basically retired from directing.
On the technical side of the business I think Lucas is a total genius; and is one of the fathers of computer effects along with Douglas Trumbell (2001, Close Encounters).