When you think of Android's openness, what comes to mind first? Is it the open source code of AOSP? Or maybe nearly 200 devices that run the Android now? Perhaps tethering, built right into the OS? How about the GPLv2 license requirement for manufacturers to publish all changes to the Linux kernel simultaneously with each phone's release?
If you are a custom ROM developer or even user, that last bit there probably occupies one of the top positions, and rightfully so - without it, proprietary changes to the kernel would remain hidden and would need to be reverse engineered.