My first serious (multiuser) system experience was VMS. The concurrency, the power of DCL, the record based nature of files, the mail box concept for process communication, the accounting system, the granular privs, the cluster concept. I came to UNIX second and still miss some of these things.
For me, the distributed lock manager, queues (SYS$ENC / SYS$DEQ), IPC via message passing (mailboxes), SYS$QIO to literally queue IO requests, event flags (SYS$SETEF / SYS$WAITFR)? Amazing clustering, - UNIX just seemed so backwards after learning VMS first! (Apologies though if I got any of those system call names wrong, its been a long time!)
