The proliferation of Power mods, especially those without care for the communities they moderated.

A mod from a community that I watched go to hell was installed as a mod in another community I enjoyed, I brought up that he failed to adequately moderate a different community, and was given a ban from both communities.

The sudden announcement and lack of transparency in their API (aka let's just do what Twitter did, because that's worked out so well) was the nail in the coffin.

I've gone back to look once or twice, I can't get to a number of my old subs because "they're not proven non-NSFW, so we must not let you see them, unless you use our app" , I've checked there when big news has dropped, and it's hilarious how bad their algorithm is screwing them, and how many bots there are in the comments when you do find the current news article.