What I've found, time and time again are the forums tend to start out great, with the whole 'let it ride' philosophy.
Then they start moderating, improving it etc.
THEN after enough people have shown up, the adverts are rolling in the coin, the traffic is climbing and climbing THAT's when it changes. The ToS changes, a new wave of moderators start coming in, recruiting their mates etc etc. Adverts start flying in all over the place to the point the navigation and speed is a joke. Then slowly over the course of a couple of years the old-guard (folks like us) are phased out via bannings, IP blocks, side-lining.
I must have seen at least three forums I was heavily involved in go down this road plus others. People migrate to other forums as well of course but the 'mainstreaming' of the internet has a lot to do with the demise of forums, PC crap etc in general.