For those that want to give a simple rating system a try, take a look at my little project GC-Vote. There currently are 25,000 individual ratings in the database, but most of them for caches in Germany.
The program only works with Firefox and you have to install the Greasemonkey extension. But the rating system is fully integrated with geocaching.com. Average ratings are displayed on all cache details, map and list pages, just as if this system was a part of geocaching.com.
I did no statistical analysis of the votes, but I have the impression that the system works ok and is better than no rating system at all.