For me, the whole turn to the dark side bit was ruined by the prequels. In the OT, the Dark Side is like a drug. Once you use it once or twice, your hooked on it and it comsumes you, and controls you. So for me, thats why Vader said things like that he "must obey his master" because he was a slave to the dark side at this point. In ROTJ, the emperor and vader are both pretty sure that all it won't take much to turn Luke dark, with Vader simply telling Luke to 'join us', or make him mad enough to use the dark side once or twice and the emperor wanting Luke to get angry enough to kill Vader. So at this point, it looks like to me that it doesn't take much to make a person dark side.

Now enter the PT. Anakin pretty much is always angry, slaughters a whole tribe of Sandpeople in a blind rage, and cold-bloodedly killing Count Dooku, and he stays light. Throw in the EU with him also using the darkside to defeat Assaj Ventress multiple times, and it seems like it is really hard to actually fall to the dark side. I guess it helped draw out the movie, but I was expecting something more quick, sudden, and not this drawn-out process that it became. I mean, something slower makes sense for something like Smallville, were we are talking about personal choices and personallity, but in Star Wars, the dark side seems to be much more of a controlling force.