hmmmm...this is a generally tough questions to answer, because it is all specifics
If we are talking about popular stuff in Japan from last season(Summer 2010): I personally liked/like Shiki, Moyashimon, Mitsudomoe, Asobi ni Ikuyo: Bombshells in the Sky, Yumeiro Patissiere(continuing on from previous seasons), and Kaasan Mom's Life(Continuing on from previous seasons), from what I have read Occult Academy is good, and Heroman is good if you like super hero stories, and Uraboku(continued from the spring season) is good if you like a more gothic anime, with some implied BL fluff.
If you are talking about what sold at NDK: FUNimation did sell all 100 copies of Hetalia, probably because they sold it and Shikabane Hime before their release date, I am sure a lot of people picked up Haruhi season 2 from the only vendor at the con selling it, though FUNimation did have a lot of great series that I think sold out its really hard to say, I think Eden of the East would have sold too, but alas the release date got pushed back.
Going by what the cosplayers were at NDK: Well its nice to see some diversity, there were a lot of Pokemon (I cant wait for Pokemon Black and White), Hetalia had its big thing, and was a nice Soul Eater crowd too, FFXIII from what I saw was popular, and probably a ton more that I might have missed.
Things that sell in America, in General: That is something only the big companies could see, and even then, I dont think they could predict what is 'popular' only what sells enough, it would take months of research to find out what is 'popular' as well as comparing what is 'popular' from one company to another.
What is popular in America, based on Cosplay: I have no idea, I think if you look at the grand scope of things, you would see too much flux in the data, because each place is different, and it all boils down to a matter of personal taste, though from what I have seen from cons around the country, I would have to say, Pokemon and Hetalia would be the two biggest things, but I dont know much, only what I see.
What sells in Japan: I haven't the slightest clue on this, I am learning, but the whole anime distribution thing, is very different in Japan than it is here. First you dont have to worry about licensing and all that. Second the Japanese can sell things more expensively than they do here, if you think $50 for 13 episodes is expensive try $~60-$70 for 4 episodes, though I think they are getting slightly better it definitely isn't to the point that it is here.
What people are cosplaying as in Japan: I honestly dont know this one either, there are two big reasons why I dont. First when you look at people cosplaying in Japan, its usually in Akihabara not at an anime convention, and they are doing it for a job of some sort. Second, Japan has more anime then we will ever, sure we can license every new anime for the upcoming seasons, but really if you think about it Japan has had over 50 years immersed in this stuff, while we have only some what, sure we both got Astro Boy, but what about all the other animes that we didnt get over the past 50 years, so if there is a popularity resurgence, and someone cosplays and that older series that is popular now, I might not know it.
