A debugger integrated into an IDE is the most important feature I'm missing. Debugging code in C# where I can step into easily, see all the code I'm in, see all the variables, all graphically, is extremely useful. In D I have to resort to printf except for using gdb/lldb to look at the stack-trace in a segfault.
Mono-D works nicely for coding, but I have not had any luck getting debugging working in it in OSX (but I use OSX Mavericks which replaces things like gcc with aliases to clang and includes only lldb by default).
