I wrote about What Is VMware Vsish back in 2010 which included a list of 771 configuration parameters some public and some hidden. With the latest release of ESXi 5, I have compiled a list of the net new configuration parameters that comes out to the following:
***As usual with any hidden configurations, please be careful and use at your own risk as the vsish interface is not officially supported by VMware***
Some of the interesting hidden parameters such as /VMFS3/EnableBlockDelete and /VMFS3/BlockDeleteThreshold could be useful in dealing with Dead Space Reclamation also known as the UNMAP VAAI primitive. For more details about the UNAMP feature, take a look at Duncan Epping's detailed post here.
For the complete list, please take a look at https://s3.amazonaws.com/virtuallyghetto-download/complete_vsish_config_500ga.html.