There's fundamentally less to do here than a lot of RPGs; less to interact with. It's difficult to shake off the suspicion that the game is a bit slight. You can get through the whole thing in little over 20 hours, which seems a bit short for an RPG - especially considering the amount of level-grinding. There's just not that much content here. The dungeons are short and simple, each of the islands is small, there are relatively few diversions to keep you entertained while plodding through the main plot, and that plot is itself pretty bare bones.
But it's an entertaining diversion, nevertheless. It's difficult to switch off Dragon Quest Monsters: Joker - you go round, collecting and trading monsters, and you use them to fight other monsters, and while you're doing that, it's pretty enjoyable stuff. So it's difficult to get too annoyed with its lack of substance when that slender substance is somehow so compelling.