Collapsible comments are available with JavaScript enabled, and the setting is remembered for two weeks, if I remember correctly.

This is little consolation to someone who doesn't have JS available, but if you are able to selectively enable JS just for HN, I have found it to be more helpful than not.

There's a "[-]" next to each comment which does this.

You can collapse the comment thread by clicking the minus in brackets next to the comment time.