Well I'm no film buff, but for me...a great film is simply one I enjoy far more than ones I consider good. Was I completely entertained? Yes? Ok...to me that's great. Did I simply enjoy the movie? Yes? Ok...to me that's good.
Then again, I really don't watch that many movies so maybe I'm less critical based on that, or, I simply don't demand too much (compared to other big time film watchers).
One thing I'm definately sure of though....if Tom was a movie, it would be super calafragilistic GREAT.