What absolutely must stay on the physical root partition?

[Ross Boylan <RossBoylan@stanfordalumni.org>]

I just added a volume manager, and now that I can I would like
separate out my filesystem into different chunks (volume; these are
just virtual partitions if you're unfamiliar with the concept).
Currently my system consists of a small root partition (running out of
space), a swap partition, and a big partition with /usr.

Can anyone tell me what / directories are reasonable to mount from
other partitions/volumes, and which must stay put for safety?

Here are my current guesses:
Safe:
/usr
/home  (since /root is separate)
/share  (oddly, not discussed in FHS)

Possibly safe:
/var
/tmp  (maybe not--if a startup process uses it, and then it gets
mounted over it seems there would be trouble)

Definitely not:
/bin
/boot
/dev
/etc
/lib
/lock
/proc
/root
/sbin

Thanks.