Cookies, browser characteristics, user behavior, etc. Just because a certain bit of information could theoretically be used for fingerprinting users, doesn't mean any system that uses that information is doing fingerprinting.
I don't know why HN seems to be so stubborn about things like this. It's clear as day they fingerprint users, but seems like the majority of users here refuse to believe a corporation or government has done anything wrong unless they literally admit it in a press release.