As for seinen, I'd recommend Blue Gender, if you haven't already tried it. However, the characters may become a problem for you in the middle of the series (in that you might find them disturbing, less interesting, or annoying). Although, at least one character still stays relatively likable throughout the series. Just be warned that this is a low-budget anime, so you'll have to be willing to look past some of its weaker animation sequences. That said, I'd still say its worth your time, since it has a good story, some decent character development, and while it is rather gory, it doesn't focus all that much on the gore when compared to the story.
As for shounen, there are 3 "manga," in particular, that I would like to recommend:
The first is Rurouni Kenshin, and even if you weren't so fond of the anime with all of its flaws (even though the 2nd arc was still great in it, IMO), the manga is much better in its execution of the story line. It also has very enjoyable characters.
The second is Claymore, which I only recently started reading, upon darklordavaitor's recommendation of it to me, but I'm already enjoying it, and its definitely a lot more interestin that most other currently running shounen jump titles, IMO.
I would also like to recommend Hikaru no Go, but that might be stretching it, since I'm not sure if it will reach your interest level. In ways, it may feel like a typical shounen, but it utilizes the common shounen aspects in a good way, making for a really interesting and addicting story about growth and development in a game that very few Americans know how to play. That said, I do know how to play the game of Go, myself, and I love it, even though I'm terrible at it. If anyone here does play it, I'd love to challenge you online in Yahoo's go lobby.