The PT is proof that directing actors is just as important as everything else.
It is important but not nearly as important as the elements I mentioned. There are plenty of films that have rather average acting but are great because of the idea/story/screenplay (for example Terminator and Predator).
And besides, it is not like PT’s only problem (or main one for that matter) is bad acting. It also has bad ideas, storyline, characters and dialogue.