I'm more familiar with the Hundred Years War era (thanks to Barb Tuchmann, mainly) and recall that taxation was a huge issue. Without taxes, you couldn't have a professional army. Without a professional army, you had trouble both defending yourself and keeping free companies from looting your own countryside when they weren't on your payroll.  You mention taxes here and how it was important. Do you recall the methods the Kings used to finally really get the nobles to consent to taxation?  If I recall correctly, even during the 12th and 13th centuries, the issue was known, but the solution evaded the monarchs.

Also interested in any thoughts you have regarding the effects of the era of Two Popes on France's consolidation.