I agree with Allronix to a certain extent. As for the difference between movie DS transitions and KOTOR-era DS transitions, I'd say that it's more likely the fact that when players play DS, they really play DS (as in max out their DS points for the Mastery bonus and full on demonic-look). DS Exile is far more darkside than say Anakin before he gets his limbs chopped off and his body totally burned, but probably just as much as Palpatine (see: Mask Theory). Dooku...I'd say he's actually not "fallen" all the way, he has certain convictions and justifications, whereas DS Exile and Palpatine don't lie to themselves (they go all out). Bastilla in K1 doesn't look like a zombie yet because she's just started on the DS road so she's probably closer to pre-Vader suit Anakin. It's all about degrees.
Also, note that the "Dark Side" and "Light Side" feature different applications of the Force. The "Light" promotes healing, increased strength, better vitality, etc.. The "Dark" is used for offensive applications such as Lightning, Force Fear, and most dangerous of all Force Vampiricy. I disagree that the powers associated with this "Dark Side" are evil (except for Force Vampiricy since it is actually an abomination that goes beyond the "Dark Side") but they are more likely to have damaging affects on the body. When using Force Lightning one is actually sending electricity through one's body (which is why Vader couldn't use it with his suit). Force Fear is probably stressful to the mind of both the caster and the affected. Force Vampiricy is...an abomination as said (I know because have studied Force Vampires and Force Vampiricy more than anything else in Star Wars). Note also that "Dark Side" powers have damaging affects on the victims but are more likely to have a steady, overall affect on the caster as the caster will have used it more than the victims have been affected.
As of 3/14/10, TSL is restored. The Sith Lords Restored Content Modification by Stoney and Zbyl has been finished and can be downloaded here