What exactly is the difference, though? Isn't the expression of an idea just itself an idea, but a more specific one?

You can't mean the physical expression, for instance, because that is the area covered by laws against theft. Copyright only covers expressions of ideas which are themselves ideas.

Great question. The short answer is it's complicated, varies by country, and there's over a century of case law with which to reference.

Here's where you probably want to get started. http://en.m.wikipedia.org/wiki/Idea–expression_divide