Simplification of the EU's primary law
The rejected EU constitution abolished all previous basic treaties in favour of a single document in four parts. This attempt for simplification was scrapped by the revised EU constitution in the Lisbon Treaty.
Instead, the Lisbon Treaty inserted almost all articles from the Constitution into the plethora of (revised) founding treaties, protocols and declarations making it very difficult for normal citizens to read and understand the “constitutional framework” of the EU.
The Lisbon Treaty has added maybe 200 pages to the existing 2800 pages of basic treaties instead of the 560 pages only in the proposed constitution.
The introduction of simplifying terms, like “laws” and “framework laws” instead of “regulations” and ”directives”, was also scrapped. Since this simplification has never been criticized, it remains unclear why it was abandoned.