I've tried Mlem and Memmy, and the biggest features missing as I see it are:

• Once you have logged in, search all currently federated servers. See what their subscriber count is on their local instance, and across all instances. Sort by subscribers, posts/day, or comments/day.

• A tab which shows you your subscribed communities so you can go straight to them (Memmy and kbin both make you go in to your profile to do this, it should be front-and-center)

• Ability to subscribe to a community by looking at its main page

• Ideally kbin communities would show alongside lemmy communities, I think this is a limitation of kbin right now though?

• Swipe posts to upvote/downvote (right), reply/save (left)

• Something which tells you the last time information was pulled from a federated server - sometimes it would be useful to know that I might be seeing a page which is 8 hours out of date vs. one which was updated 30sec ago

• Something which tells you or prevents you from posting to a defederated community since the post will not behave as expected

• Expose options for copying links to comments, links to parent comments

• Ability to see your post history separated by threads/comments/etc., and messages; from a comment, go to the specific thread in question (i.e. direct link to parent or contextual comments in that thread, not just the original post)

• Hide posts you've voted on already

• I don't think any currently existing apps expose moderator tools. I'm not sure how much of this is present on the API side so far but will be hugely important as communities get bigger.

• Content density. On Apollo I can see ~6 posts at once on any given page. On Memmy or Mlem I see ~2.5. Just a much more efficient display of content, tell me what the post title is, what community+server it's on, how old the post is, how many comments, what the upvote-downvote calculus is. If I want to know the user count or read the blurb on the post I can tap in to it.