As with fears about Episode VII, there is no reason why a good PT couldn't have been made.
It just wasn't.
The key is to replace any backstory you expose with more backstory.
Who is Palpatine, where does he come from? Is he a politician who becomes a Voodoo Freemason Wizard or a Sith pretending to be the universes first honest politician? There's an interesting backstory there which should have been hinted at from day/episode one).
What are the Sith anyway? What is their history with the Jedi? Why do they want revenge?
Also making characters we don't know the final fate of the main characters would allow the story we know to happen but not spoil the episodes that already exist. It would also create drama because we don't know what will finally happen to them and the writer isn't handcuffed to material we have already seen.
So really Anakin, Yoda, Obi-Wan, Palpatine, The Droids, Tarkin, The Jedi, should be secondary or minor characters (but they should be characters and not some bloke wandering around in an Halloween mask in the final sequence).
If expectations were played with it could have been great (the remake of Night Of The Living Dead worked this way, things happened differently creating new drama based on familiarity with the existing material).