Having just read the summary linked to above in full, what lets the movie down are these points:
-- 'Luke' is a nice guy, out to get revenge for his mother. Apart from doing well on test scores, I see no similarity between the real Light - who wants to become a God, caring for no-one - and this goody-goody 'Luke' fella, who simply wishes to kill bad people.
It's a more extreme version of what the JP live-action team did in a poor attempt to make Light (needlessly) into a more sympathetic character. Stop with the bastardization of one of the best characters ever created! Light is Light; no more, no less.
-- Instead of being faithful to the manga, the movie is more faithful to the JP movie, with added American cheese. The whole girlfriend plot fuckery from the first JP movie is included, and 'Luke' even saves a woman at the start for no apparent reason (at this point).
-- Ryuk doesn't even appear. This means, as well as no decent humour, it being replaced with cheese, there won't be any talk of trading half of your life for the eyes.
-- According to a comment I've read, there's no 'L with Asperger's syndrome quirks' mentioned in the descriptions of him. Like Light, he'll probably be nothing like the real L.
-- The end sounds really, really bad. Light loses his memories, gets away with it and continues living with regret. If you're going to end Death Note, either have Light win or have him die in dramatic fashion. Any 'in-between' ending is a major copout.
...Basically, instead of staying true to the source material, those in charge decided to be more faithful to the first JP live-action flick, also adding in some good ol' Americanization/cheese, ruining Light's character by doing so.
Why, oh why, do the idiots in charge of these movies insist on adding girlfriend characters (as in, characters Light/'Luke' actually cares for in return) when Light never gave a **** about anyone, never having one real friend and simply viewing women as objects to be used? It's ******* moronic. For a movie to work, there doesn't need to be a bloody romance going on.
And, if you're going to change the ending, copy the second JP live-action flick - which has the best ending - or just end the story in a random warehouse. 'Happy ever after' just doesn't work with Death Note, period.
/mini-rant end