At this point, I should back up and explain. The relevant terms here are shonen and shojo, meaning boy and girl, respectively. Shonen anime is marketed towards a predominantly young male audience, shojo towards a predominantly female audience. Naturally, there's a lot of crossover, anime is anime

I won't go farther than that, though any serious otaku certainly would. In my world, anime is for boys and girls, and that's that. Now, even though these two market catagories are very broad and overlap in many places, there are some definite differences. For example, shonen anime often contains some kind of fighting or battles. While the same is occasionally true of shojo anime, the battles in shonen anime are tactically explicit, meaning that a good deal of time is devoted to showing exactly how the hero wins (or loses) each battle. That is markedly different from shojo anime containing battles, where it is usually underlying strategy or alignment which determines the outcome of battles. Clearly, it isn't as simple as all that, but the distinction is evident when you take a series as a whole.

Now to the meat of the discussion. One characteristic (and predictable) difference between shonen and shojo anime is the degree to which anatomical differences between main characters are emphasized. In shojo anime, young male and female characters look pretty similar, differing mainly in clothing, hair-style coloring. Minor characters may have exaggerated physical differences like oddly shaped eyes or anatomy, and adult characters are more clearly male or female in body type and facial structure, but the distinctions don't usually go much beyond that.

In shonen anime, it's just the opposite. All the main characters are very physically distinct in anatomy, and the differences between males and females are clearly evident even in pre-adolescent characters. Now in shonen aimed more at a teen+ audience, there will usually be at least a few bishojo characters, meaning hyper-attractive young females. Depending on the context, these characters might actually have less exaggerated anatomy than other female characters in the same series. The important thing is that they are always drawn a bit more carefully so they look good.

Back to shojo anime, you naturally have bishonen, hyper-attractive young male characters. If you have enough of them and some of them are the main characters, then it's a bishonen anime. Now the important thing to note is that while bishojo characters are hyper-feminine, bishonen are never hyper-masculine. Both words use the mei (ӟ) character, implying a more feminine beauty. While bishonen clearly have a masculine body type, they are always slender and delicate rather than muscular. And their faces are generally indistinguishable from those of females, particularly in terms of the nose and chin. It is also worth noting that all the bishonen designed by a particular artist will have nearly identical body types, varying mostly in height, where bishojo will always have very distinct body types with lots of obvious variation.

Clearly, all this says a lot of interesting things about how men and women naturally want to view the differences between the sexes and between individuals. But really I'm just introducing these concepts as a prelude to a specific theme I noticed when I started thinking about bishonen.

You see, bishonen and characters influenced by the bishonen aesthetic appear in shonen series as well. Probably the least (or most ) ambiguous example is Sai from Hikaru no Go. With a gentle contra-tenor voice and delicate features, as well as his archaic clothing and make-up, a lot of fans had trouble telling whether he was a man or a woman. Obviously, he's a guy...but he's also a bishonen. Another major character with clear bishonen influences is Touya Akira, with his somewhat feminine hairstyle and delicate facial features. Turning to a show like Bleach, we see Kuchiki Byakuya. Very manly, but clearly a bishonen, actually prettier than most of the female characters. An interesting point about these bishonen, they display an important characteristic of bishonen in shojo anime, their "manlyness" derives from extremely advanced abilities rather than from masculine physiques or voices.

Is there also an observable difference in the way these shows portray people of other races and cultures? Also, are hyper-masculine/muscular types relegated to the position of dimwitted henchmen?

It would be interesting to subvert the stereotypes and see what happens (other than failure in the market).

Edit: PS. You can probably tell the only anime-esque stuff I've ever watched is Spirited Away. So am pleading ignorance here.

I'm female, and I enjoy shonen manga. It was the first stuff I watched & read, other than Inuyasha which has points toward being both shojo and shonen. Romance and action...shojo and shonen, basically. Now, I prefer shojo manga, but I still love shonen manga, too. I buy both Shonen Jump and Shojo Beat.

I don't get the question, but it's a fun topic to discuss.