To me, it was the astounding amount of interactivity between the community.
At first I thought this was temporarily caused by the whole migration from the R site. But, just out of curiosity, I signed up to Mastodon and have enjoyed myself just as much as here.
Most of the Lemmy post's / Mastodon toots have almost as much or more comments / boosts than upvotes or favorites. It feels so organic and makes me realize how much these huge companies employ technics to pretty much force to interact the way they see fit.
It reminds me of that good old saying "you are not immune to propaganda", well I guess neither I nor anyone is immune to psychological tricks either.
P.S. I also love the fact that since there isn't pretty much any money involved, most opinions and interactions are genuine. Like, who is gonna pay this dude to advertise a book through BookWyrm? That increases immensely the odds that said person is being honest with their opinion of that book. It's amazing.
The two biggest surprises I've had so far:
I hate to even suggest it just because of how much of a buzzword it has become. But blockchain feels like a possible answer to the identity problem. It would couple one's identity to the network as opposed to the instance.
That's not to say that instance-level identities shouldn't be allowed as well; but it would be nice to have the option. Right now one basically needs to sign up for separate accounts on as many instances as possible to prevent bad actors from posing as them. A universal ID would solve that.
Regarding the first issue, you could check out https://hubzilla.org which has NomadicIdentity, but it still doesn't solve it for ActivityPub. Maybe later.