I have been doing quite a bit of elisp hacking lately, for some bigger project. Multiple buffers must be coordinated, as well as some external process. Although my elisp is slowly becoming more idiomatic, I find that it offers little to help with modularity, information hiding etc., so it all comes down to self-discipline.
Having an interactive environment is nice, but can also be rather confusing, i.e. how to ensure that the function that's being called is the one I want, not some old version that still lives on in the netherworld.
Some of this is clearly the impedance of a brain wired for C; but still, some of the glue for modularity that e.g. Guile delivers would be very welcome.