What's strange is that I'd lump josei and seinen together before I lump either of them with their younger demo counterparts. Mainly because the focus on the younger demo-oriented works are so much more far-flung when you compare them, with MCs hyper-fixating on romance in shoujo and MCs usually fighting or doing sports or some other kind of action/adventure in shounen.

But as you get to josei and seinen, there's more of a slice of life pace and work & mature, nuanced romances take the center stage. Probs because this is what people relate to more as they get older rather than going on adventures & the follies of youth. The presentation is different, but not by much, in my opinion. Seinen tends to be a little heavier on the sex angle (since it's male-oriented, I guess) and the responsibilities of men in society (youth and regret are big themes in seinen) while joseis have a lot about the place of women in society in general & balancing a family, love, and work and if it's possible.

As a josei lover, I loved Sekitou Elegy and Haipoji HIGH POSITION. Seinen has the leg up on story-telling and I love the character development seinen offers. I am reading Kuu Neru Futari Sumu Futari and really loving it, it reads a lot like a josei but it has that ripple of messiness that seinen doesn't shy away from (messy in the sense that people fart, the apartment is dirty, they smoke casually, drink a lot -- in josei, there is always such an idealized lifestyle and if the apartment is messy, it's a sign of something being wrong/chaos in the MC's life rather than normal). I am also a huge fan of 1122 although there were some aspects that could have been improved in the story telling, it was still very real to life. This one stood out because although seinen is for guys, it was very much the female MC who was the focus in 1122.

The shounen I like are usually the popular ones, like Saiyuki (how I was obsessed with this series, omg) and Yu Yu Hakusho. I also liked sports manga with, ahem, big BL-potential like Tennis no Ouji-sama and Slam Dunk. I was also into High Speed! (Novel) (also known as Free!) recently. You can see the trend. Lol.

The thing I've come to realize about all these genres though is that there is little consistency and a lot of diversity. You can kind of see that with what people are recommending even in the same genres. Some shoujos are fantastic, some shounen are as shallow as the worst shoujo, some josei are just shoujo with older characters, and some seinen are just perv fodder. The substance is what matters -- a well written description and the reviews from fans (can't stand critic reviews) are what I go by before I decide to check it out; the genre is just a way to get an idea of how the story might be presented. I'm willing to give everything a shot.

http://i.imgur.com/LxhWm.jpg (will reformat this some day…)