The manga is pretty dark. Tsukune dies quite often in very brutal ways. Inner Moka gets thrashed around quite a bit. Moka's sisters are batshit crazy. The Crying Freeman parody sister is a bit creepy. The monsters in the manga are actually pretty brutal and there are a couple [mildly] disturbing elements in the story. And Mizore's arc is kinda sad, I thought. I didn't particularly care for the Jiangshi arc, I thought it was weak. Inner Moka gets more time on the page in Season II though, so that's nice. I like Inner Moka more. But Mizore's my cup of tea. And holy shit she's fucking horny. For a girl so icy, she warms up to a guy pretty quickly and things heat up fast.
To summarize, it's a bit like Kekkaishi -- it starts off pretty weak, just some fan service and stuff, then shit gets serious near the end of the series. That's how a lot of manga is. If you start a series and like it a lot in the beginning, odds are you won't like the end. If you start a series and think it's weak and cliche in the beginning, it probably gets good in the end.