Since its inception in early 2009, has this community ever (1) changed its collective opinion on something (2) had a debate in which there were two sides and there was a clear victor?
The annual prediction threads, and the website PredictionBook (built by Trike Apps and heavily used by LWers), showcase a lot of arguments about empirical predictions that have been resolved.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.