From the download page: "Although we like Disconnect, Adblock Plus, Ghostery and similar products [...] none of them are exactly what we were looking for. In our testing, all of them required some custom configuration to block non-consensual trackers."

"algorithmic and policy methods for detecting and preventing non-consensual tracking"

https://www.eff.org/privacybadger

So I think the biggest difference is the algorithm it uses to "learn" what to block as you browse.

Edit: Also, it doesn't block all ads. From the same source: "Because Privacy Badger is primarily a privacy tool, not an ad blocker. Our aim is not to block ads, but to prevent non-consensual invasions of people's privacy"

Ghostery should really be replaced/removed from your machine if you value privacy.

I used to be a Ghostery user up until I realised they're playing gamekeeper and poacher.

https://en.wikipedia.org/wiki/Ghostery#Criticism

Ghostery doesn't seem to be open-source.