Right now I just want the bug-free stability, and the well developed communities for every niche subject imaginable, but I'm sure those will come in time.
I'm using Jerboa and the only thing I miss is the "view context" button on my notification messages, and the option to "view parent comment" when viewing an individual comment. Searching and linking between instances needs to become more seemless too, and maybe better integration with things like Mastadon, although I don't use it personally.
I also think up/downvote history being public is a terrible idea and would like to be able to disable that in an account's privacy settings, idk if that will ever be possible.
(Edit) Also the ability to migrate your account and posting history to a different instance, so that users can easily vote with their feet if their home instance gets defererated or makes an unpopular decision. Again idk if that will ever be possible or not
I think simple is better, and Lemmy already does everything I liked with Reddit. Heck, it actually does one thing even better: I love being able to sort by New Comments.
A solid way of searching communities, integrated with third party apps. Good autoplay and video controls for most video sources. A reliable way of hosting videos that would be anonymous and easy. What is bothering me the most right now is a bug, subscribing to communities in other instances requires pressing subscribe twice.
I understand they're trying to be different but there's really not enough delineation between posts and replies and it's hard to keep track of what's going on. Is this a reply to a post above it, is this a new post? If there was more indentation and the color bar was a wee bit thicker that'd be wonderful. The colors in particular are not super helpful to me and probably should be avoided because of color blindness. I'd rather see tab lines like you'd see in like vscode instead. I understand this is up to the individual federations/instances with their themes(maybe?) but almost all of them go with this really subdued style that's hard to read.
For browsing on desktop I really miss the Reddit Enhancement Suite browser extension. Continuous scrolling, being able to choose how links behave, dragging across images to enlarge, being able to rotate images, speed controls for gifs, just generally being able to customize how the site works so that it's most convenient for me: RES just made Reddit so much nicer to use. I really hope someone builds something similar for Lemmy.