Personally, the first impression I have of Miyano Mamoru's voice for Light doesn't have a tinge of evilness at all. Maybe it's due to the fact that I've been having this impression from reading the manga that Light is a deep guy who has lots of clever ideas within without showing them on the surface but I can understand the rationale for making Light sound like an ordinary teenager right from the start so as to show the development of his character from one who believes in justice to someone who becomes too entrenched in his beliefs and goes wayward.
As for Nakamura Shido's potrayal of Ryuk, he sounds quite close to the impression I have of the character which probably suggests why he wasn't dropped from the movie or anime despite his recent scandal of drink-driving. If he wasn't that good, I think he would have been replaced under normal circumstances since that's what usually happens to celebrities who get into trouble with the law.