I myself will have to check out some of these suggestions, great thread.
Also, while I won't pretend to be an expert on what is and what isn't seinen, +1 to BTOOM.
While I haven't actually watched it, the manga's pretty good and I haven't heard any serious complaints about the adaptation.
Nekomonogatari
(a prequel to Bakemonogatari, which was mentioned earlier) just aired recently.