First, let me say that I didn’t even read it. The thing, despite its name, isn’t really a constitution. It’s rather a long and complex treaty reorganizing the european institutions, and in particular the decision making-process, due to the extension of the EU from 15 to 25 countries. It has been first drafted by a (non elected) convention, and then modified and watered down during lenghty negotiations between the member states. The main issue was what would constitute the qualified majority required to adopt an EU regulation. The result is quite complex (a given %age of the countries representing a given %age of the population, with at least a given number of countries voting for it, a minimal number of countries required to constitute a blocking minority and some other subtleties).
It also settled (temporarily at least) issues like the number of members of the European commission, which decisions have to be decided at the qualified majority, unanimously or left up to the member states, somewhat reorganized the (turning) presidency of the Union, created a " EU ministery of foreign affairs" (sort of), drew the limits of the co-decision power of the EU parliament, included provisions about the decisions concerning the Eurozone, allow member states to “go ahead” alone if they want to implement a stronger integration despite the oposition of a minority, fuse the treaties which created the various european institutions (common market, european economic community, EU), and a number of other things.
So basically, it’s mostly a new treaty reforming the EU institutions, which was necessary since the current rules were unoperable with a much larger number of countries, and would bring the EU to a complete halt. Of course, it includes various articles and provisions about rights, or the color of the EU flag, and such things, but still it doesn’t deserve the name of “constitution”, especially since further evolution of the insitutions will be necessary in the future.