technologies you should know as a C++ programmer (and therefore more technically knowledgeable than lesser programmers ;) ):
performance issues - what makes things go slow, how to find and fix such issues. I also mean stuff like context switching, cache lines, optimised searches, memory usage and constraints, and similar stuff that your average VB/C# developer doesn't care about.
threading issues - how to get the most from a multi-threaded app, how to detect and fix abuses of the same.
low-level communications - especially being able to connect to obscure systems that no-one else has written a toolkit for (especially radio comms), latency and bandwidth management.
Otherwise, for specific tools - it depends on what you're targeting, Windows dev will be different to Linux, different to embedded.
