The biggest difference I've experienced between the fediverse and Reddit is how infrequently I come across outright bigotry in the comments. I didn't expect that, and I certainly didn't expect that to feel as refreshing as it did. I had gotten to the point where I was just numb to the racism, misogyny, homo/transphobia, etc. Not having to put up with it to nearly the same degree in my first few weeks over here was amazing.
Now that I've (a) seen that a community can exist with a minimum of people calling me slurs, and (b) realized how much those comments were actually affecting me (I'm not so tough after all), I am very aggressive about protecting this space while it's still possible to stem the tide.
I suspect a lot of other Reddit migrants feel something similar. We've been forced to put up with these chuds for years, to the point where we've just started treating them as an inevitable, natural forces, like shitty weather or beach seagulls. Over the last four years, particularly, we've very clearly seen the result of allowing these fuckfaces to take root and grow their communities. Now that we have a chance to make something different, a place where that doesn't happen, we're (pretty collectively) jumping at the chance to actually fight against them. Trying to fight them on Reddit was like trying to fight a snowstorm, or fire ants, or the just the fucking tide. Here? There's a chance.