I literally fell asleep playing Hyperdimension Neptunia. Full on, snoring for thirty minutes while my characters bobbed up and down in battle stances waiting for me to input their next attack. Now I will admit it was late, but I have never been so disinterested in what I was doing in a game that I've fallen asleep before, and that includes some very late-night romps through other turn-based JRPGs.
There are two things that people always talk about when discussing an RPG: the world and story, and the battle system. Sometimes when one is really good, it can make up for the other. An RPG with a crappy battle system -- but a great story -- is still playable because you're pulled through it by the narrative.
Likewise an RPG with a bad story and a great battle system can be fun simply because beating up enemies is fun. Unfortunately, both aspects pretty much fail in Hyperdimension Neptunia.