The main difference is very simple, yet effective: HN has a specific goal. It is not a place which will just flow with whatever is posted. Some things are allow

The main difference is very simple, yet effective: HN has a specific goal. It is not a place which will just flow with whatever is posted. Some things are allowed, and some things are not. Hopefully, the choice of content will keep people interested in that kind of content, and remove those that don't. They already got reddit/digg/foobar for their stories. Comments would take more time to moderate, if we think of users up/downvoting as a differential equation that slowly is moving towards the "intelligence of the masses". So in time, the level of comments will degrade. The way of keeping that from happening is, of course, to mercilessly kill bad comments and keep them out with overpowering ;)