This is what matters for me. I want my program to have acceptable speed just by writing it in the most straightforward way. Squeezing out the last bit of performance isn't usually necessary, but 10x slower is also unacceptable. Many people defend slow languages by saying that premature optimization is the root of all evil, but in C/C++, you simply don't need to do ugly premature optimization in most cases, since the program often runs fast enough as long as all best practices are followed.
C++ seems to be useful in most cases as long as all developers know it well (using correct data structures, not copying large objects around unnecessarily, etc.), including important libraries in STL and Boost.
C is also good with the right libraries such as glib. It is more verbose than C++ and manually freeing all the memory can take a bit of developer time, but it is also easier to learn.
Fortran 90 is even easier, but is only suitable for numerical programs without complicated data structures. Java is also easier and fast enough, and may be a good choice for non-numerical work (numerical programming is still a bit awkward).
Scripting languages are, for the most part, still too slow for non-scripting work. Sure, a program in these languages can be made faster by writing some speed-critical parts in C, or by writing the program carefully so that my particular implementation can optimize it well, but this will usually take more effort and make the program harder to maintain.