Decided a few weeks ago to give it a re-read for the first time since it came out.
The series gets retarded after L dies, but Light was retarded from day one. that's why L's death is the breaking point, I think even to the author. After Light "outsmarts" L the world becomes unbelievable. for all the stupid shit going on, that's the breaking point, because it fucks with the internal logic of the world.
Read the first couple of chapters to remind you, youll se what I mean. Light's a self defeatist from the get go.
Also, having Light sit and yell at a computer screen while everyone else gets shit done for an entire arc. He's no fun as either protagonist or antagonist when he's completely without agency, and when he's neither, there's nothing left to engage you. No other character besides L has any character without Light to react to.
It's funny how his motivation is to become a "god of the new world", when he lives in a world where he is litteraly the center of everyone in it. dude won before he even begun.