Main character that has next to no flaws
Love aspect is incredibly rushed/forced, especially in the case of the side-characters
Plot holes such as no one noticing anything fishy about some dude that's practically capped all his stats at the release of an MMO, people trying to blackmail money in town when other players cannot be hurt, logically stupid plot points such as sleep-PK.
Lack of setting exposition, and due to the lack of there are plot points that are pulled out of the ass just because it's vague
Contributing to a flawless main character, other characters pitifully die in one or two hits without a fight, or are extraordinarily bad at the game/makes bad decisions and weak beyond belief
Off the top of my head. I'm sure half of it is solved with the LN.
Wasted setting potential is probably the biggest disappointment. Good idea but...executed so poorly, and so little is done with it.