I suspect that one problem on Twitter is how wide you "open the door". If you crack it open just a little (i.e. limited follows), you can have a nice, useful, informative stream there, without all the negativity. But if you insist on opening the door fairly wide (i.e. lots more follows), you're effectively inviting the whole world in.
And guess what? "the whole world", taken as a whole, isn't so great.
And guess what? "the whole world", taken as a whole, isn't so great.