Immaculate UI/UX. Whether in landscape or portrait orientation, there are many clever flourishes: it uses Tweetie's "pull up to refresh" mechanism for moving between articles in a feed; or you can swipe an article left or right to return to the feed index; or you can pinch the article/feed to return to all of your feeds. In a feed index view, you can swipe a list item to the left or right to either toggle the read state or the starred state.
The feed index is designed very similarly to the iPad Photos app: you can either pinch open a folder to view the feeds within it separately, or you can tap it to view all of the articles in a combined view; similarly, you can pinch open a individual feed to preview of the articles within it.
Additionally, Reeder caches articles and images for offline reading; and the top-level feed organization is pretty sensible, too... it's separated into 3 sections: read, unread, and starred. It's also extremely fast, having been rewritten to use SQLite instead of CoreData.
