You get crappy URLs with closures; no other web framework provides ugly URLs like that -- but if they did -- they could do it with the same amount of effort. But routing in rails is specifically designed to separate URL presentation from the underlying action.
Hell, you can get routing for free in PHP if you just name your files like: comment.php, topic.php, upvote.php, downvote.php, etc.
Users don't care what the URL looks like, Amazon does quite a bit of business with its crappy URL's and closures allow linking to actions without having to have a resource for every action. This is extraordanaryily useful when building complex applications.
Look, I use both styles daily and I'm telling you it'll be over my dead body before I allow someone to take closure style ugly links away from me. You aren't going to convince me that manually routed URL are always preferrable.
Users who bookmark and search engines care about non-random non-expiring urls. The urls on HN are wrong in every way a url can be wrong.
Closures create a resource for every closure instance which isn't very scalable or efficient -- it is in fact the problem with this very site. It might be useful for building complex applications but it's just a liability and a waste for something as simple and busy as HN.
I'm trying to convince you that manually routed URLs are always preferable but they should be for a site like this.
Hell, you can get routing for free in PHP if you just name your files like: comment.php, topic.php, upvote.php, downvote.php, etc.