Theory: If Anakin's decision to join Palpatine as his apprentice is "understandable", this movie will fail.
Rationale: As of A New Hope, Darth Vader is introduced as the embodiment of everything evil the Empire represents: uncaring, cold, merciless, stupendously powerful. Upon walking onto the screen for the first time, we fear and hate him based solely on his frightening appearance (and that sublimely intimidating Williams score). The following few minutes of screen time enhance our fear and loathing of Vader. He's almost unbelievably bad.
I don't want that opinion changed by watching episode III. I need to see Anakin choose hatred and jealousy over love, Palpatine over Obi Wan, and the promise of great power over his Jedi duty. I don't want to think to myself "I don't blame you, anyone in your shoes would have done the same thing, Ani." For me that will ruin the character as much as taking the decision to kill Greedo out of Han's hands (sorry) spoils Han.
ASIDE: (who I also think needs to be seen as ruthless and cunning to work properly. Leia loves the bad boy, and can't understand why. That's the whole conundrum from her perspective, why minimize it? It's even more suitable now for her to love the bad boy - her mother fell in love with someone capable of slaughtering an entire band of Tuskens, did she not? Like mother like daughter. Sorry, an argument for a different topic I guess).