One thing in this thread that I don't think anyone has mentioned yet is how some trademarks have been lost through their falling into common usage.
Aspirin, dry ice, e-mail, laundromat, petrol, thermos, videotape and zipper are just some of the words that were once protected trademarks but because they eventually came to be seen as the common name of the product or service by the public lost their legal protection as trademarks in the U.S.
There are other names that are still protected trademarks in the U.S. but because the public often uses them in a more generic sense the trademark owners usually have to spend a portion of their budgets fighting to keep those names protected. Some of these would be Band Aid, Xerox, Coke, Google, Clorox, Jell-o, Kleenex, Popsicle, Walkman and Vaseline.
BTW, did you know that DC and Marvel have a co-ownership of the term Super Heroes?
Best
Joe