Unfortunately, I just can't read discussions that are collapsed by default. Way too much clicking — some threads have hundreds of comments, often deeply nested.
I also think you could spend some time on colour and font choices. It's faded and hard to read, with almost nothing prominent. HNBuddy (https://i.imgur.com/cEPrInb.png) vs MiniHack (https://i.imgur.com/ED4YXte.png). I'd like the title font to be more legible, and the number of comments should be much more prominent. I often decide whether to read a story and/or thread based on the number of comments.
I do like that there's a way to get to the comments after clicking on a story. MiniHack doesn't have this.
Offline, opportunistic fetching and caching of comments would be good. Like the sibling poster, I read HN on the subway, where there's spotty cell service.
As an aside, since you're not charging for this app, perhaps it would be a good idea to make the code open source? I've been through multiple HN clients, and they all tend to stagnate over time. Putting the code on Github and accepting PRs would help avoid this.
[1] https://itunes.apple.com/us/app/minihack/id631108846?mt=8