I can't speak for everyone, but I really enjoyed the "get out of your way" style the app has. I also really liked that you had to slide to upvote/downvote on posts instead of having static buttons you can accidentally press. It also had a decent system for switching between multiple accounts. Finally, it's all open source so one could fork it and customize it if they wanted to.

Custom color-coding of subs was nice when you scrolled so far you forgot where you were.

I really like it has buttons to jump to previous/next parent comment. It made skimming top comments a breeze.

I liked that it was available from F-Droid.

There were a couple of other Reddit clients also available from F-Droid (Dawn, Diode, Infinity, RedReader), but I don't remember why I picked Slide over those. All I know is that I didn't spend much time evaluating others and that Slide was "good enough" to not annoy me into wanting to switch.