Psyringe: I did read some superhero comics as a kid, but found them very boring, and unnecessarily violent, and the dialog seemed abhorrently crude.
I tried some other "mature" comics which had been recommended to me by friends, and some of them had pretty interesting settings, but all of them seemed to favor the depiction of simple gore over good writing, so I wasn't too fond of them. If someone knows a comic that actually has great writing, complex plots, and original ideas instead of the usual "pile up the violence, the readers will like it" paradigm, I'm open to suggestions. But currently I think that nobody cares to do a comic for people who share my preferences. ;)
I wonder what kind of "mature" comics did you try? Did you ever read anything by e.g. Art Spiegelman, Will Eisner, Chris Ware, Craig Thompson, David Mazzucchelli, Joann Sfar, Daniel Clowes, Jiro Taniguchi, Charles Burns? (The work of the latter does have some explicit content bordering on the grotesque, so you might not like him, but he's the exception to the rule.)
Depending on your preferences you could also give these ones a shot (they don't pile up violence, in any case, at least not in an explicit and exploitative way ;) ): Lewis Trondheim, Manu Larcenet, blutch, Killoffer, Gipi, Igort, Guy Delisle, David B., Adrian Tomine, Jessica Abel, Derk Kirk Kim, James Sturm, Marjane Satrapi, Alison Bechdel, Howard Cruse, Aaron Renier, Keiji Nakazawa, Reinhard Kleist, Isabelle Kreitz, Mawil, flix, Arne Bellstorf, Ulli Lust etc. etc.
(Note that most of them tell stories that are either related to real life experiences and aspects, some fictional, some biographical, or philosophical, grotesque, absurd, or humorous. They rarely are fantasy, sci-fi, action comics or similar, if that's what you're looking for.)
EDIT: The Kozure Ôkami (Lone Wolf and Cub) mangas by Kazuo Koike and Goseki Kojima are violent but pretty good story-telling, if you don't mind that. These short stories about a dishonored samurai who works as a contract killer try to depict a realistic image of the Tokugawa era in Japan, and they often have surprising twists.