Bluntly saying POSIX is too restrictive seems over generalized. For some cases I am sure that is true, but again, would you ask you local filesystem developers to ignore POSIX? Would you expect them to agree that it is too restrictive. If you start with the mindset that you want distributed FSes for things that are typically done on distributed FSes, than it will likely seem restrictive. But if you start with the frame of reference of things that are typically done on local FSes, and you simply want to migrate to a distributed FS without recoding applications or worrying about consistency issues, then I can assure you that POSIX semantics become very important.
-GPFS, very nice.
-Lustre, does it provide HA, or just fast (stripped) access?
-GFS (I assume you mean RedHat's GFS, and not google FS) is not really a distributed FS, it requires a shared backend.
-Panasas, is this really Posix (hard to figure out form their site)? pNFS, yes. Also, this seems to use their custom hardware on the backend which happens to be distributed, a black box so to speak. So, this is not really a distributed FS that others can install and use.
So, it seems like possibly only 2or3 solutions and there are many many more distributed FSes out there. I would say that overall posix is quite rare for distributed FSes. And since none of those rare exceptions are free/libre/open source, surely you would concede that ceph is not filling an already crowded space?
None of the proprietary solutions are things that your average company is going to install on their desktops which likely have virtually unused TB drives (except for the possibly large mp3 collections scattered here and there). There clearly is an opportunity to harness tons of corporate wasted disk space and CPU into a shared pool instead of wasting good money on obsolete high end fileservers which typically don't even provide HA.