The GPL has a viral clause that says that whatever code touches GPLed code must be GPLed as well. This is a deal-breaker for many people who may be working with code under different licenses without similar viral clauses already in their codebase. They don't necessarily have the right to GPL other people's code, so they have to forgo using the GPLed code.
It's not just the GPL, this criticism applies to any licence with a viral clause.
I understand that as an author I have copyright and ultimate freedom to licence my code under whatever I choose, but I choose to release my code under non-viral terms.
It's not about posting changes back, I don't care if they fork it and run I just want people to use it. I see viral terms a huge roadblock to this type of creativity.
So there's a solid argument for permissive licenses that excludes viral licenses like the GPL!