To @molf's list I'd add:
DSLs - how to use them, and how to write them. Digging into the internals of rake and capistrano can be really useful for this.
Rack - a key advantage of using Ruby (vs. Python, particularly) to do web applications. Understand why it's an advantage, how it works, and how it's typically used.
Message passing vs. function calls - another key advantage of Ruby over something like Java. The use of method_missing and how it makes ActiveRecord and DSLs possible.
