It also means that anyone can make their own instruction set extensions or just some custom modifications, which would make software much more difficult to port. You would have to patch your compiler for every individual chip, if you even figure out what those instructions are, and what they do. Backwards, forwards or sideway (to other cpus from other vendors) compatibility takes effort, and not everyone will try to have that, and instead add their own individual secret sauce to their instruction set.
IMO, I am excited about RISC-V, but if the license doesn't force adopters to open their designs under an open source license as well, I do expect even more portability issues as we already have with ARM socs.