Here's the thing. I'm a mod for a small-time community for a niche interest, !castles@lemm.ee I'm also on Mastodon, and was before my Reddit exodus. I follow #castles as well as a few other related topics on Matsodon, so I get quality toots, such as this: https://mastodon.scot/@McNige/110926238926867959, that I wish I could just crosspost over to my community. Currently, I have to repackage the toot, which isn't a huge problem, but currently I just drop them a note on Mastodon that their content has been posted elsewhere on the Fediverse. What would be nice is if people who comment on the Lemmy post also get fed into OP's toot. More sharing, more connection, more activity.
On the flip side, I've subscribed to @castles@lemm.ee on my Mastodon instance and, while it's good to be able to follow posts in feed form, it looks like ass: 
I realize I should try this with Pixelfed, but I haven't made that leap yet.
I don't know, am I thinking crazy here? I'd think we'd want everything in the Fediverse soup interoperable in a more seemless way. Is this a feature request or am I missing some way to do this better?
It's totally not crazy thinking. :) I think the main problem is that while Mastodon and Lemmy implement the server to server part of ActivityPub, they don't implement the client to server part of the standard, and instead build their own REST API and client. This is why, while you can subscribe to actors from an other application, it looks bad : it's supposed to be consumed in their own client, or something that tries to emulate it (that, and the fact that they each implement their own extensions to ActivityPub, it doesn't help).
In a perfect world, ActivityPub based applications would implement the client to server part or the standard too, so that we have a multitude of third party clients that can consume data from any ActivityPub based application without looking broken. I certainly hope we go in this direction in the future, because interoperability looks half-baked, as it is right now, and the fediverse would be just more awesome with such upgrade.
Nah that's not it. It's just AP is extremely broad and Mastodon only cares about a hyper-specialized subset of it. AFAIK even with things like writefreely (which serves Page objects as opposed to Note objects which are what Mastodon uses for microblog posts) it first "converts" them to extremely long Notes and then handles federation that way.
From what I know other implementations such as the *omas or Friendica are a bit more lax and generally have wider compatibility with non-microblogging AP.
At least I'm not crazy. Glad you agree.