My list:

1. Assembly programming. Not one particular assembler, but the whole concept, and being able to learn a new dialect by reading the datasheet.

2. Programming in C. This is pretty much standard.

3. Real-time systems. Seriously. The one single class from university that proved most useful.

4. Some digital design skills. Otherwise, you're "just a programmer".

5. At least some basic system theory (time domain vs. frequency domain), control engineering and signal processing. (Yes, you might be required to do the latter even when there's no DSP involved)

6. Math. Actually, that should be the top of the list. Numerics, algorithms, number theory.

7. At least basic analog design skills. They're handy, even if you're working with a dedicated electrical engineer who handles the whole hardware part for you.

8. Debugging and problem analysis strategies. How to use oscilloscopes, logic analyzers, signal generators and various other lab equipment. Getting a knack for making a good initial guess on where to look for a problem.

9. Reading. To be able to deal with all the datasheets. :)