I actually think it's the bad scenes that "save" the prequels. The "sand" dialog and the love dance in the flower field, those sorts of things at least make the movies funny in a "Plan 9 from Outer Space" sort of way. The periodic cool visuals and unintended comic relief combine keep them from just being boring.

So the fatal flaw? Boring. Why? Uncompelling story and wooden acting. Why the wooden acting? Probably because of bad script and poor direction with too much use of bluescreen.