The complaint most people have is that the interface sucks on mobile phones. There is no way to expand/collapse threads and hitting the up/down vote arrow is impossible.
However, I exclusively use http://ihackernews.com on my phone which solves both problems.
I find it interesting (not in a snarky bad way mind you) that HN is the source of many great startups pushing the envelope with what can be done with websites and yet here we are on a site that uses tables for layout, perfectly functional and in use.
