It would be nice if the previews would work without needing an API key :( - could you do a readability version and an iframe version?
one niggly visual thing! In Chrome (windoze 8 Version 40.0.2214.115 m) When I hover my mouse over the images in those 'cards' a new full height scrollbar appears on the right.
This is beautiful. Add keyboard controls and it'll become my favorite way to read HN.
My only other suggestion is to remove the title bar above the article preview, since it pushes the content below the fold and doesn't contain new information
A recommendation for you: read more than a couple of words before clicking to another thread to hate on - "Setup Readibility integration to enable article previews."