Cameron would have given a much greater sci fi feel to the movies. A greater purpose if you will. The "boy and his car" theme, the robots as side characters, the sex jokes, the parents, tuturro, etc none of that would have been there. Basically cut all the cheese out and replace with a better focus.
After watching that scene in avatar where the bad guy uses that robot, I was convinced he could have made TF a really great movie.
My only pick above Cameron would be Blomkamp(sp?) from District 9 fame. I think he could do a REALLY good tf move.