Everything about development and the OpenMW source code.
Since this has seen renewed discussion, I'll contribute my two cents -- as a mod user, albeit an advanced mod user hoping to eventually become a mod maker, I would vastly prefer separating content from resources. A way to set the order of folder loads in a GUI in much the same way as rearranging plugins would be nice, as might an automated or GUI way of adding files to openmw.cfg, and perhaps deactivating them via GUI as well, but quite frankly, I've come to really, really like the current approach, and working with it has made me much more comfortable with manually editing configuration files. I love being able to switch pluginless replacers around on the fly without impacting content files at all by simply rearranging lines in the cfg, especially. Compacting things into archives and associating them with content files would be really, really annoying from my point of view. Seriously, I'd view that as a pure, unmitigated downgrade.
I agree with Jemolk. At this point the only real thing I could see myself using is a GUI drag and drop + click to activate system to automatically fill the config, but even then filling the config manually is so simple and allows such control of what's installed that I don't really see a manager as necessary.