From what I can gather it isn’t true that Reddit culture has completely supplanted what came before, but it has definitely shifted things overall, both mixing to some extent. Scale is part of that though, as is the filtering mechanisms provided by a relatively niche platform.
Antagonistic downvoting (I’m now basically against downvoting I think), superficial statements, especially those that are dismissively in disagreement to the point of unpleasantness or abusiveness … I’d say I’ve seen more of all these things.
One effect, I think, is the establishment of Reddit replacement communities and their gaining large membership which has shifted the centre of gravity here. The whole of lemmy.world being an example.
Besides all of that, I’d say I’ve seen the generally or more frequently presumed set of “obvious” opinions shift toward the mainstream, which isn’t surprising at all, but with a slightly ruder and superficial form of engagement (at times at least), it’s rather tiring.