I would like to suggest one thing concerning the success rate of levels:
There should be some way to tell the difference between impossible
levels and levels that have so low rate that it rounds to 0%. For example
my Ultimate Defence level had 0% success rate when it started getting
more plays and only I had won it once, so I had to play it again to get it to 1%.
Maybe something like Super Serif Brothers has; an indicator that tells whether
anyone has successfully completed the level or not (ie. if it's impossible or not).
There has been discussing in one of the other threads that one of the requirements before saving is that the author must beat it at least once. This would solve the problem fairly adequately imo.