Does granting SA rights to a user in SQL compromise the OS?
sysadmin rights allow the use of things like xp_cmdshell and the sp_OA% stored procs which can affect the underlying OS, but it depends on the service account used

If you limit the service account permissions (eg not local system, not local admin and certainly not domain admin) then it's pretty much OK. The SQL installer limits the permissions of the service account during install, of course this assumes you don't escalate permissions yourself.