Downvotes, or as they called them "burying a post." Basically, in v4, they redid a large portion of the site, with many many bugs present, a UI that the majority of the users hated, and made it a better platform for advertising instead of making improvements to help the community. It was a problem that came about in part from them working towards making it something profitable (ala advertising) versus something the community built.

Here is a link on reddit that discusses it a bit more in depth than what I'm able to, since it's all second hand on my part.