It is always a bad idea to start charging people for things you used to give them for free. It is always much better to give them new paid features, while not pissing off existing user base.
Starting not from too long ago, these LJ bastards start requiring paid account to see blog statistics. What the fuck, dudes? WordPress does this for free, you did this for free. Does your Hadoop cluster need an investment?
P.S. And don’t get me even started about lack of new features.