Are there specific packages that you would currently need, but aren't there (yet), or is it a more general concern about the relative package scarcity and unforseen changes to / future needs of your code?
mmap is a package that let me create a fast db from scratch in a day. ggplot, lattice and all the charting routines are a joy to work with.
I'm interested in random forests today, and I bet there's a package that links to the standard libraries whatever they are. This is also probably true for python, but Julia you'd have to build the API.