Smart^ people don't get banned on Reddit they just get downvoted.

People come to HN comments to have a constructive debate, so they are less likely to downvote for disagreement. I often upvote comments I disagree with if the point is well made.

People come to Reddit to have their beliefs confirmed. The subreddit structure reduces mixing between users with different beliefs, and where that mixing does occur, e.g. on the large subreddits, it tends to turn into a downvote battle.

^ It's not really about intelligence but openness to new ideas. You can have one without the other.

does HN even have a downvote button?

For comments, yes, for stories, only flag but not downvote.

But both flag and downvote are karma-gated (I think the threshold is 500 for both, but I could be wrong.)

Yes, but only for comments. And it's limited not only by karma, but time: It's only available for a rather short time after a comment was posted. Dunno how long exactly, but feels like something between... 12 and 24 hours?

"HN mods are lenient". Maybe. But I've be down-karma'd so much I've had to create half a dozen new profiles just to stay above water. I'm a retired senior engineer from aerospace (so, not an ignoramus), I am very curious, I ask a lot of (what I think are) intelligent questions, and invariably I get downvoted (or whatever) for unknown reasons (since downvotes are invisible to me), So I just figure there are a pile of butt-hurt simps here, along with VERY intelligent and curious people. Weird mixture that I'm not used to. In engineering, I got along with virtually everyone. Not so much in this wild-west internet space...