The recently released Blu-ray edition of Star Wars has a few changes made personally by George Lucas. The most striking is Darth Vader's "No!" in episode VI. Why were these changes made, and do such changes affect viewers' understanding of the plot?
I would like to just sum this up as the jumbled effect. Lucas basically wants as much action going on in his movies as humanly possible. Adding attention to items that we already know are there. This can be seen more vividly in Star Wars 1-3 where he adds more characters and objects to the set.
A lot of inference is lost because of this. Instead of asking oneself why an object is on the screen just offset from center, we have the object in plain view center focus, distracting from what is important, establishing the plot and following through with that plot.
Items like these distract the viewer from what is actually going on.
The most controversial change with Darth Vader, changes the emotion and environment of the scene completely.
To who? himself? the emperor? the dark side? to his son dying? Too many questions it was better left mute and let the background music take care of adding in the emotion as well the turning point to that scene, making that scene appear longer than it really is. Now that "no" extends across this entire section, the turning point is now a sharp inflection leading the viewer wanting the scene to be over as quickly as possible.