Hello LJDawson, let me start with saying how happy I am to have Sync back :)
The one issue I have with Lemmy is duplicated posts between different communities (see screenshot). Posts usually have same title, author and link, yet differ in community/server they are posted to. It would be great to be able to make the appear as one item on the feed with some note below that links to both instances' posts, or something like that basically to make them not appear twice or thrice one under another.
Yess, I hide read posts, but tons of duplicates appear in my feed anyways. This is probably a bigger problem and will be even bigger with scaling on Lemmy. So it'll be interesting how we solve it. Maybe group together federated posts that post the same link in a 24 hour period? You could open a post and it'd say something like "also posted in X" and you can see all the similar posts. This would be a lot of work on Lemmy's end too though. Just food for thought.