What kind of phone or tablet? It works great on my iPhone / iPad, better than most mobile sites. I know that the Android browsers zoom differently than Safari on iOS, maybe that's the difference?
You can't vote (you have to zoom 3x or you'd accidentally down-vote a poor guy), and the front page is not readable in portrait mode, and only barely legible in landscape mode. Nested comments (more than 4 levels deep) are usually unreadable.
One rather handy feature of Chrome on Android is showing a small, zoomed window of where you click when there are a few links together; allowing you to select the correct link. I also assume this feature is present on the iOS version of Chrome.
Google "Responsive Web Design." you shouldn't have to pinch your zoom your way into every website. HN has a poor mobile experience.
Comment pages look okay on your iphone when there is six or seven levels of replies? I switch to iheckernews.com if I want to read a long comment thread on the iphone.