The main problem is that the amount of reward per unit of time investment put into the game has dropped off dramatically since the early parts of the game. When I stopped playing, I had reached a dungeon called Shinra Tower, where a number of problems in the game design came together to make the game a lot less fun than the preceding parts of the game:
1) The amount of story has dropped off. For most of the game, there was a story scene before and after every battle map. However, the only plot so far in Shinra Tower came at the beginning of the first battle of the area. There have been several battles in a row with little plot.
2) It is hard to tell how much further I have to go to reach the end. Since Shinra Tower does not follow the typical pattern of four stages per game chapter, I don't know how many more stages I have to clear to get to the end.
3) The levels of the enemies began to increase at an accelerated rate. This means each stage requires more leveling up to be on an even level with the enemies than earlier parts of the game.
4) There is no area where it is easy to level up to the necessary level range. There are maps that are designed to help level up your characters in Disgaea 2, but the only available ones have much lower level enemies than the enemies in the higher stages of the Shinra Tower.
5) By the time I had reached this part of the game, a new wave of brand new games had already been released. So, there were other games I was interested in drawing my attention away by the time I was late in this game.
All told, this point of the game has become a larger level grind than earlier parts of the game, paired with a dramatic decrease in story reward, and no clear end in sight. These are all results of the game's design, and they all conspired to make me lose interest in playing more of Disgaea 2. The only reason I want to finish the game is thanks to an emotional investment in the characters.
Unfortunately, this situation comes up a lot in console RPGs for some reason. For example, Final Fantasy 12 lost my interest when I reached the Phon Coast, where I had just passed through three whole game areas with little to no plot, the enemies had become so much higher level than me that I had no choice but to stop progressing and level grind, and I had no clue how much further I had to go to reach my destination.
Any place where an RPG forces the player to level grind, rather than progress at a reasonable rate through the story, is simply faulty game design. Some of my favorite RPGs are my favorites because this is not a problem in the system. For example, it is possible to beat most of Xenogears without ever getting off of the rails of the plot in order to level up (excepting the second disc, but the problems with that part of the game are both famous and too numerous to discuss here). It is possible to design an RPG where the player does not have to invest any game time level grinding. So there really is no excuse for RPGs that do force the player to do so.