One thing I wish mhddfs would do by default, is to keep leaf nodes grouped together on the same underlying drive/filesystem. Eg. If I have a directory called "Top Gear", then ideally all Top Gear episodes should be kept together within that directory on a single underlying drive, rather than potentially being scattered across multiple underlying drives.

Dunno, why, but that's just how I'd like it to work, and so I've patched my copy here to do exactly that.