Documentation or self-documenting code is important. For example, for the nvec driver (in staging), I have added code documentation (mostly kernel-doc-style function documentation) and am now working on moving the magic constants and stuff into enums with readable and understandable names.
It looks fairly out of place compared to the rest of the kernel, but since we don't have any documentation on that device at the moment, and implement the driver in the community, having the documentation makes it easier to understand the driver and fix bugs.
We also have some strange things, like having to add an udelay() somewhere because the device otherwise locks up, and we don't know why. NVIDIA's original Android driver did not have those things and worked. Maybe there documentation has information about this, but we don't have it (although it might be made public if we're lucky).