0. Nice minimalism.
1. The main body typeface is needlessly hard to read (words have weird kerning with some letters sticking together a lot). I strongly think that the body font should be as readable as possible, simply since it is going to be read a lot! Pick the most readable font out there (or maybe the second most readable).
2. The "Return" button is hard to tap when holding the iPad in both hands. iBooks solves this problem by having the entire left edge tappable; not sure what would work best in this app.
3. Are you familiar with the Flipboard for iPad? (you probably are). I think it is beyond awesome. All magazine/web mobile apps should learn from that app.
4. That every article pops out from the bottom edge of the screen (UIModalTransitionStyleCoverVertical ?) gets annoying after a while. The animation where the whole screen whizzes by from the bottom up every time you tap is more tolerable on a smaller screen like the iPhone. I would rather prefer an animation with a "zoom" effect with the article expanding from its location in the grid to the entire screen; or maybe something else you can think of.
5. It is annoying that comments have to be shown in a browser window which takes a while to load.