I have the impression that as Thomas has added more "special" dungeon features (statues, shifting rooms...) then other existing features (vaults, shops) have become progressively rarer. I saw a couple of these rooms occur right after they were introduced, but haven't seen any since. Maybe it's time for a revision of the code that picks these features.
Overall, what percentage of unscripted levels should be 'special' in some way? 1/3?