This is my beef with the new Sparrow Gmail client on OS X as well. When I open up Gmail in Chrome I can see 33 messages at once (40 once I scroll). By contrast Sparrow shows me 13 (always utilizing the full vertical of the screen). Now arguably it presents them in a better fashion (although the label display is lacking, but I digress), but is it worth sacrificing 2/3rds the visibility? Not with my email volume.
Granted, information density is less important for HN, but then so is aesthetic appeal (the last thing I want is more people joining HN because it has an elegant appearance).