I think losing the ability to lurk in active communities of people I disagree with politically. I would never engage as it didn't seem right, especially down/upvoting, but It was always interesting to see things from the other perspective.
So places like r/tories, r/conservative, r/prolife, r/liberalgunowners - as a guy from the UK a lot of the subjects discussed in a few of those didn't affect me but I always prefer seeing things from both sides (even if I would never agree with them)
I think we stand a chance of bringing over the best parts of reddit while leaving behind all the negative aspects, I think a lot of people who lurked before (myself included, <4000 karma after 10 years) will be much more willing to engage here.