Depends where you go. For instance, the discussions in AskLemmy seem better than the discussions in AskReddit right now, but lots of communities are basically empty still. The main baseball community on Lemmy.ml only has a few highlight videos posted, pretty much all from the same user. The corresponding Reddit feed would be much more active with both posts and highlights. It’ll take a bit of time before enough people migrate over to start posting content again.
Agree that it shouldn’t be so complicated. I see that as a major flaw of the platform that will curtail adoption, but who knows, maybe one will win out over the others?
In any case, my understanding is that you can’t log into the other instances with your username from lemmy.one, but you can read posts and interact with communities on different lemmy sites. For instance, I’m commenting from lemmy.world on a post you made using lemmy.one at a community hosted on lemmy.ml, but we can both read each other’s comments, and so can people that signed up on other instances like beehaw.org.