We have been one of those forums affected on Nov 16, 2012 (yes, 3 years ago!) with a very big drop overnight (from 800.000 unique visitors before that to 400.000 now).
Since that time we were not able to grow anymore in traffic, in fact we end every year with less traffic.
There was an interesting discussion here about this problem, which affected a lot of UGC sites.
And on last year this change was even confirmed by Matt Cutts due to the buzz generated by the Q&A site MetaFilter.
Things we have done on those 3 years without any noticeable effect
Noindexed tag pages
Noindexed member profiles
Pruned old threads
Added rel="next", rel="prev" for pagination on threads
rel="nofollow" for external links
Removed threads below certain word count.
Based on the 'ugc' context what else could i do to recover from this past Google update or at least improve the SEO of a forum?