[*]Being able to treat a pointer as an int (0 or >0) to test whether it's valid or not.
[*]Separate header and source files
Besides, have you ever written cross-platform C/C++ headers? I have and the amount of platform-specific hacks necessary is ridiculous (this was a C++ OpenGL header with proper namespaces, enums etc).
I am just waiting for C++0x to make its debut, to improve the C++ syntax further.