Katekyou Hitman Reborn
I'll start off by saying the only reason why I keep up with this series is because it's the favorite series of one of my closest friends and he likes to talk to me weekly about new developments. He prefers the more recent story arcs in the series and doesn't understand the dallying in the first eight volumes when it was episodic and not continual plots. For me, it's the opposite because of this fact: It was episodic and silly for eight volumes. That was the set-up. A bunch of weird incidents that Tsuna has to deal with; he's basically the voice of sanity in a increasingly crazy environment.
That being said, for the series to suddenly become serious is a complete turn-around. I cannot be serious when the set-up was silly. There're babies running around, Tsuna's main weapon is a bunch of mittens, one of these said babies has an egg for a head and a weird countdown... You can't just switch genres and expect everything to fit. I mean, you can be comedic and have serious plots at the same time -- two popular series like Bleach and One Piece are like that -- but Katekyou Hitman Reborn was not like that. It was lighthearted through and through. And to change the mindset from that is difficult for me.
Plus I find the plot to be kinda weird, but that's for another day.