Pointer arithmetic. Returning multiple values from a method (through pointer variables). Creating objects (actually structs) on the stack instead of on the heap. Manually cleaning up memory*. Communicating directly with the OS and hardware platform.
* my onetime real-time Java project could really have used this. With little strain it ran for a few minutes, then crashed with an OutOfMemoryError because the garbage collection could not run.
I miss it, although I still do enough embedded work that I don't have to miss it for very long :) What I *really* miss is making dynamic, extensible language implementations in resource-constrained systems a la Piumarta-style stuff, which makes use of pointer arithmetic. Quite enjoyable.
If there's one language I truly miss working with, it's Forth, although I may take up Factor again. On an only vaguely-related note, I did get to work with actual PostScript for awhile when working at a high-speed printer manufacturer. In addition to the language, we also had really fast cameras to take pictures of how paper moved through the printer--before then I had never really given such thing any thought: paper physics. Crazy stuff.