Arguably, the biggest mistake Light made was one of abstract strategy: he started using the Death Note almost immediately after obtaining it. He should have spent many years testing the thing, pondering its implications, studying police work, etc, before putting his plan into action.
I can't help but think that that represents a serious privileging of the hypothesis - given a little black notebook claiming such absurd powers, you shouldn't carefully devise 20 different studies which try to falsify your various theories and inferences about its powers & limitations.
Unless you mean that after he verified that the Death Note did in fact kill supernaturally as claimed (after the biker and hostage-taker, I suppose), he should have gone into scientist mode?
In that case, my first thought is that from Light's perspective, delay is massive ...
I don't know if this is a little too afar field for even a Discussion post, but people seemed to enjoy my previous articles (Girl Scouts financial filings, video game console insurance, philosophy of identity/abortion, & prediction market fees), so...
I recently wrote up an idea that has been bouncing around my head ever since I watched Death Note years ago - can we quantify Light Yagami's mistakes? Which mistake was the greatest? How could one do better? We can shed some light on the matter by examining DN with... basic information theory.
Presented for LessWrong's consideration: Death Note & Anonymity.