FYI, it also fixes some bugs in Vanilla and G&K (for some reason, the "G-word" trips Newegg's wordfilter) that will not be fixed without the expansion, so you're essentially throwing money at them to add back cut content and bug fixes.
It adds things back to Civ V that should have been there in the first place: trade routes, great works, the world congress..; etc.
They're basically charging $30 for features that were in Civ IV; that 2K probably made Firaxis remove so they could sell it as "DLC" later