A user @supercomputing figured out a bug in the mining algorithm that allowed him to pretty much take over the mining rewards system for himself and booting everyone else out.
Mining is crucial in attracting new users from the crypto-currency community. I myself came here originally to learn how to mine steem. If the mining system doesnt' allow everyone to mine easily it will really hurt Steem's growth.
Right now it's pretty much impossible to mine thanks to this hack being exploited and they have also removed the market maker liquidity reward system due to abuse. That's two core reward systems currently broken.