Seems there's less of it to enjoy when you're 32 than there was when I was 16. Hardly surprising considering the vast majority of it targets Shōnen (typical boy from elementary to high school age) or Shōjo (young girls of comparative ages).
Note, I said majority of it, there's certainly a good-sized adult-themed Manga audiene as well, which might surprise some Westerners. Personally, I've a guilty pleasure for dōjinshi and other ecchi comedy, though hentai stuff is (by definition) wantonly perverted brain poison so I try to abstain from that as much as possible.
Other bits of manga I won't touch with a ten foot pole are yaoi, romances featuring two males, which apparently they make a ton of because the aforementioned Shōjo think it's sooo romantic.
If you get deep into manga, it can actually be a bit dangerous if you live in a Western country, because there's a great deal of Lolicon and Shotacon favoring cultural sentiment. In Japan, being a very young (ambiguously pubescent) girl is considered sexy to the point where older women try to imitate them [http://en.wikipedia.org/wiki/Lolita_fashion], and this saturates much of the material which can get you downright prosecuted to have in your possession.
Thus, if you live in a Western-thinking country, you should demonstrate a certain reservation if you don't want to be seeing the inside of a courthouse, like this guy [http://www.wired.com/threatlevel/2009/05/manga-porn/]. You're safe enough with the vast majority of what you'd find on the shelf in a Barnes and Noble, but you could be playing with fire if you're working directly with importers (e.g. J-List [http://en.wikipedia.org/wiki/J-List]).