• A comment and post draft system. You can save drafts to restore them later. It's really nice when writing an "effort comment" (kinda like this one) and you want to resume it later.
• When submitting something, it saves a temporary draft in case the submission fails. Several times I wrote a long comment in another Lemmy app and then lost the entire thing when the submission failed.
• A ton of customizations, so many options and view modes. Customizable actions menu. Excellent contextual button.
• Cleanest interface of any app I've used.
• Biometric lock for opening the app or changing accounts.
• Masked usernames in the account switcher for privacy when showing the app to someone.
• Can pin favorite communities at the top of the subscriptions panel.
• Separate settings per account. (Kind of a drawback sometimes. It really needs an easier way to sync settings between accounts.)
• Backup all your settings to a file. (Which can be used to sync settings between accounts.)
• Can submit a comment as a different account than the one you're browsing with. (Useful for a quick reply which you don't want to make as your official account. I like to keep stuff like politics out of my account that's associated with my real-person online presence.)
• Long-pressing any setting in the app copies a link to that setting to the clipboard. When pasted in a comment or post, other Sync users can tap the link to go straight to that setting. Useful because there are so many settings!
• All replies to a top-level comment can be collapsed by default with an option. This is great because the top-level replies are the most important, and then you get to choose which conversations to open.
• Each comment has a button to jump to the patent comment, and long-pressing that button jumps to the top-level comment of that thread. This is excellent when a comment thread has hundreds of replies and you're done with it and just want to collapse the entire thread and read the next top-level comment.
• Speaking of comment navigation, the reddit version had a handy, optional comment navigator UI. ~~Not sure if it's here yet.~~ It's there!
• You can search settings. Really nice to find A specific setting fast, or all settings related to "comments", etc. I just used it to determine that the comment navigator does exist in the version and I just turned it on.

I could probably think of more if I spent more time on it.

Liftoff was the best alternative app I used, but it's not as customizable. It also had a confusing account switcher. In fact, you didn't switch accounts at all. You switched instances and had a default account per instance. Switching accounts on one instance required you to open the settings and change your default user for that instance and then switch to that instance. It was even more confusing because you were allowed to change users when viewing the Inbox, but that had no effect on who was actually logged in. I made several comments under the wrong account while using the app because it was so confusing.