Have you seen the tweetbot app (http://tapbots.com/software/tweetbot/)? A recommended download. Swipe left, Swipe right on a tweet perform actions. This sort of thing is perfect for this app.
Swipe left: Comments
Swipe right: Article in browser
Double click article: Read Later in Instapaper
Great app, and congrats. First time I've taken the plunge on an iOS HN app, and not disappointed so far (a little afraid though, due to potential drop in productivity).
I also really like the idea of double tapping -- maybe (instead of Instapaper) for the article directly, as a shortcut to avoid loading the comments if that's not what you want.
I think the details toolbar is more useful where it is since I generally don't flag or upvote an item unless I read it. But that is a great idea (the swipe-for-options). You can include adding to Instapaper, and perhaps twitter/facebook/email sharing.
Also, a small bug: All the comments on the submissions say 0 points.
Nonetheless, the app is a great start. Glad to see it on the AppStore.