You need /etc to be RW, and you need that very early in the boot sequence, so it traditionally has to be inside /. That's the only thing that gets in the way of mounting / RO.
Posted Apr 9, 2009 23:37 UTC (Thu) by giraffedata (subscriber, #1954)
[Link]
You do have to use a rather different filesystem layout than what comes with conventional Linux to have a read-only root filesystem. I've been doing it for years, though. Sometimes it's as easy as having symbolic links from the root filesystem to a read/write filesystem, but other times it's as hard as having to modify a program to eliminate a hardcoded file name.
I also do daily automated backups. Deleting my home directory wouldn't be particularly severe for me.