To start, trademarking a word is not like Copyright or Patent law. You're not claiming invention, or ownership of the word. What you claim is that people associate that word or symbol with your organization. For example, Livejournal. "Live" and "journal" are good, old fashioned words. Putting them together was mildly inventive, but strangely, I can use Livejournal as much as I want. I can use it in conversation, I can use it in my posts, on websites, in books. The only thing I can't do, is create an online service that uses the name "Livejournal". Why?
Because, an unwary consumer might, mistakenly, think that my site is LiveJournal. Think about it- when you're looking for information about White Wolf roleplaying guides, what's the URL you think of? www.whitewolf.com, right? Of course, that's not the right site. It's www.white-wolf.com- but no big deal, Whitewolf sells software, White-Wolf sells books. No confusion.
Here's the thing, when the average person hears the word superhero, do they think of Marvel and DC? I have to say, I think the answer is yes. In terms of market share, they rule the roost, and the average consumer is probably unaware of anything but characters owned by Marvel and DC. And, just like LiveJournal, there's only one thing I can't do- and that's make a comic book, or other periodical, with the word Superhero in the title. They can still use the term inside the comic book- they're not trading on the name Superhero, and that's all that's important.
Somebody in the Digg comments referred to Cory Doctorow as the Trotskyist Doctorow, which I found rather amusing. I still like reading BoingBoing, but y'know- some of the criticisms are right.