- a majority of "won't boot" bugs are of the form where
it fails to find
the root filesystem. Quite often this ends up being a busted initrd
for whatever reason. There's not really a good way to debug this
other than to pull apart a good and bad one, and see if the bad one
lacks modules that were in the good one for eg.
It would be nice to have the initrd /init shell grok some kernel command
line option to enable -x style output for the initrd, so you can tell
exactly what commands were run and compare that to output from a working boot.