Perhaps about the quality of the comments. In my opinion it's not really that bad but reddit-style comments are often cheap shots that are slightly off topic, some of which I see present here
But that's exactly what the HN guidelines are trying to prevent/discourage -- meta discussion about Reddit-quality comments.
It doesn't add to on-topic discussion.
There are many differences IMO. Moderators, for instance, censor a ton on reddit; but the UI is also different. I much preferred old.reddit.com over both the new reddit (which is utter trash) and hackernews (which is simple, but also ... strange and awkward, much more confusing than old.reddit.com - and I hate the fact that after like 5 comments, I get locked out for hours before I can make more comments, that is the worst decision hackernews ever made).