Heres what I think with my detective ability:
1. Since Zac Efron will be in it they are going to try to play off teenage girls, pre-teen boys, and the uniformed anime/manga fans. This will mean that to get the preteens into the theater, and also their teenage sisters so they can oggle Zac Efron they will get the rating as low as possible by cutting corners etc. They will cut out Ryuk because hes a "Scary Image" they will cut out the death montage because that will get them up to an R, a pg-13 if they keep it cleaner than a privite bathroom.
Disney Stars + Teenagers - Lower Ratings
2. They will make alot of plot changes. Any of the good Death Note fans have seen a clip/trailer/or the whole movie. So they will be trying to keep it the same but make if different...as expressed earlier. They have changed the names so they will try to Americanize it for the audience, which will screw up the plot a little bit.
Original Ip - Plot Screwups - Americanizing
3. IT IS ANIME. I saw Dragonball Evolution in theators, this will suck about the same. Hollywood can't do shit for anime. They never can, they probably never will.
Greedy Executives+ Misunderstanding the IP
4. Making characters PC, and friendly. They will ruin Light and make L seem like hes crusading for justice, not just doing this because hes bored and has nothing better to do.
/Controversy
So heres what it comes down too
(Disney Stars+ Teenagers - Lower Ratings +Original Ip - Plot Screwups - Americanizing + Greedy Executives + Misunderstanding the IP) /Controversy = A Shit Film, much like Dragonball Evoultion.