Several hundred mutually-dependent git repositories, held together with a set of python scripts, and a healthy dose of luck...
I'm a noob with source control systems, and currently I only know entry level Mercurial stuff - enough to get by on my own solo project. Still, it surprises me how little support there are in source revision systems for sub-projects. Basically if I want to add a subproject to my project, I need to copy+paste it into my project, or else plan to deal with loads of pain. Every programming project is built upon other projects that are built upon other projects - sometimes only a few, sometimes dozens or even hundreds.
Having proper sub-project support would actually increase my willingness to A) stay up to date, and B) contribute fixes back to the original project repository.
It'd also be nice if there was even support for having a .git repository as a sub-project of a .hg repository or vise-versa.
Easily, I mean. Right now I could probably set one up, but it'd require too much manual configuring and potential for human-error.