The main features of the reddit app I used (joey) that I enjoyed:
More condensed/streamlined interface with less wasted space compared to the official app. Also much faster and more resource efficient with imperceptibly short loading times for text posts.
Ability to set custom filters to automatically hide posts with a given keyword in the title or subreddit name from my feeds.
Way better built in image/video viewer compared to the official app.
Option to move the title bar to the bottom
Subscribed subreddits shown as tabs in the title bar with the ability to swipe left and right to switch between them.
The feature I miss the most: anytime you opened a post or followed a subreddit link, you could swipe right to instantly go back to where you were like the back button in a browser. So if I clicked on the subreddit name from a post on the frontpage to open r/aww, opened a post in r/aww, and clicked on a link in the comments to open r/illegallysmolcats, I could then swipe right and be back where I was in the comments, swipe right again and be back where I was in r/aww, then swipe right again and be back where I was in the frontpage. And this stacked indefinitely so you could be 15+ subreddit links deep and still go back to where you started in a few swipes.