I discovered an amazing amount about myself - like most other people have said. For me, it was more discovering things that I thought I had "dealt with", that in reality I'd just buried. Opening to poly made me confront those things head-on, and while I'm still dealing with them, it's making me a better person.
It's things like separation anxiety, insecurity, etc. I'm dealing with them head on now, when before I had CLAIMED to deal with them - like I said, I'd only buried them.
By proxy, it's made me more anxious, more insecure, more possessive in the mean time. lol It's changed me for the worse in the present, but will make me a FAR better human being in the future once this shit is out of the way.