[rwmj]

In libguestfs (part of our open source virtualization tools suite):

- the ability to mount VM filesystems on the host via the API, which was a huge amount of work for a fairly small gain, mainly wrangling FUSE (https://rwmj.wordpress.com/2012/03/29/libguestfs-mount-local...)

- a way to make it easier to use libguestfs from Linux distros other than our primary ones (ie. other than Fedora, Debian): http://libguestfs.org/download/binaries/appliance/

So two deceptive features that are small, but involved a huge amount of work and wrangling behind the scenes, particularly the first one. Made much harder by the primary requirement to write most things in C.