I really don't like the default behaviour of su on 10.0. It's very "un-SUSE-ish". And it's very easy to forget, until things don't work.

From the release notes:

Becoming Superuser using su

By default, calling su to become root does not set the PATH for root. Either call su - to invoke a login shell with the complete environment for root or set ALWAYS_SET_PATH to yes in /etc/default/su if you want to change the default behavior of su.