Ah. Once again, my technique has been utterly destroyed by a higher level of anime-jitsu. *bows* My respect, sensei.
...Though, what with simplistic names like "shonen" and "seinen", you'd think I'd know this.
Shounen is manga/anime intended for young boys, and tends to imply the usage of certain tropes and character types. YuGiOh isn't the best example... DBZ is more classic shounen.
Seinen, conversely, is any anime/manga series intended for adult men, and this covers an incredibly vast range of genres. I usually only go for the out-there stuff (Haruhi would be included in this), or really, really well-done slice-of-life like Azumanga.
So, is the difference between shonen and seinen like the difference between Yu-Gi-Oh! and When They Cry? I'm only aware of the differences between shonen and shojo and shojo and mahou shojo.
I'll probably finish up Haruhi by watching Sigh before moving onto what I want to watch. Definitely going to check out that Baccano! thing though.