Much respect to Ted, who is awesome, but he's definitely incorrect about SMP being a major reason for the ext4 decision. SMP is also not the driving force for eMMC storage device choice -- that's more of the usual cost/performance/density decision making typical to storage parts selection for embedded devices.
As far as making platform design decisions more visible, that is something that we do want to do, and we are also working to move more of the core platform development "out in the open" over time -- for example the SDK tools development now happens entirely against the git repositories on android.git.kernel.org.
More documentation, aimed at lower level platform developers, OEMs, and integrators is also something that we're working on, especially around the HAL modules and other points of integration with specific devices.