Robert_Charlton - 9:06 am on Dec 4, 2010 (gmt 0)

Could you elaborate on what you would consider a "clear and unambiguous structure"?

Have a clear taxonomy and hierarchical structure.

what sorts of things do you see people do that might sabotage their structure?

I often see people putting too many unrelated links on a page in an effort to reduce click depth. Having too many choices isn't clear to the user, and it isn't clear to Google. In general, what works well for the user works well for Google.