I think the biggest thing people don't understand about copyright is that, for the most part, the corporations that hold the copyrights on most major works (film, books, music, etc.) STOLE those rights from the creators either through unfair business practices or straight theft. My experience is limited to the movie studios and record industry, but these companies claim copyrights they do not hold all the time. A lot of DMCA takedown notices are invalid on their face because the company making the infringement claim doesn't actually hold the rights.
This beyond the fact that every single modern work is completely derivative of older, non-copyrighted works. Ex. Disney claims they own "Snow White", a story that is hundreds of years old. It is literally impossible to create a new song. There are only so many combinations of notes and they've all been used before, there hasn't been a "new" song for hundreds of years.
One could also talk about the enormous damage copyright does to history and culture. Since nothing goes into the "public domain" anymore, that means that modern works (tv shows, etc.) will simply cease to exist after a few decades since nobody can legally archive them (except the corporate owners WHO NEVER ARCHIVE ANYTHING) and even illegal archiving is technically blocked (DRM, etc.). Most films from the 1930s through 1970s are completely gone for this reason, they only exist in a few private collections (if they exist at all) and they can't legally be shared or distributed to anyone.