I think there are several valid definitions for "advanced" in this context. While basic awk/sed knowledge is certainly useful -- if you can do the same thing in python/ruby/perl that might be fine.

Knowing a little bit about writing valid ksh/bash shell scripts (being at least aware of bash-isms) would certainly be useful for sys adm work.

My opinion is that the full gnu (and/or bsd) userland has grown so large over the years, that saying that you should know it all is probably overkill -- but knowing how to look things up (man, info) is a must.

As Zed's implied -- GNU documentation could stand to be improved vs bsd man pages -- although the info manuals can be quite useful.