i hated L's voice. it was too deep! it has to be higher pitched and more boyish. also innocent but confident at the same time.
I can say the opposite for Ryuk in the Japanese version. His voice was too deep and had really no personality (from what I've seen), but his American translation has a voice that fits his character model, more like a personalitive voice.
I really don't see so many problems that you Japanese purists do with English voice overs. Death Note has fine American voices, yet a lot of you guys just want to point out the flaws.