For me, that sense of completion comes from beating the Big Bad ones. The games almost always have a "final boss" type of monster, and often stronger ones waiting beyond that point. It feels satisfying to reach the point where you can comfortably take on those challenges.While I'm all for a more passive method of stoytelling (otherwise I wouldn't be such a huge Dark Souls fans), it's still important to me for there to be a story that you can finish and then feel some level of completion about. Without that, Monster Hunter just feels too much like Grinding: The Game.
There is a lot of grinding, yeah, but you only need to grind for the things you want. Other than that, it's all about the pure fun of the gameplay. That's why I can play the games for thousands of hours and not get bored, but I know that not everyone has to enjoy that as much as me or the same way as me. Still, I think if you're looking into playing a game like MonHun, I'd expect you're in it for the fun of it above all else. That's what MonHun is mostly about.