(Yes, I know what that implies about mine, too.)

The first was when posts started showing thumbnails. This made image content much more viable. Unfortunately, while there's plenty of awesome image content, it's also one of the lowest-effort routes for "shitposts" (the definition of which is left to the reader).

The second was the growth of subreddits. This allowed the community to split and silo itself. As a clichéd example, you can pick your left-wing politics subreddit or your right-wing politics subreddit, and you never have to mix with the other, bad one!

And the third was the death of Digg, which sent a huge influx of people Reddit's way. That one's got a lot of folk mythology behind it and little hard data, so I'll say no more.

Which is mostly the opposite of HN, which skews older and actually more educated, comment guidelines are much stricter, and the topics covered are much more technical and narrow. Even non-tech topics covered on HN (e.g., in-depth discussions of particular judicial rulings) aren't really of much interest to an average teenager or a person in general. Which leads to a few different potential effects:

a) Older and more educated people have much less reason to "flex" how smart they are. The also have fewer occurrences of delusions of grandeur/Dunning-Krueger effect.

b) Niche technical topics have a barrier to entry, which already eliminates a gigantic portion of the toxic audience. The topics themselves are also largely uninteresting to an average person.

c) Stricter comment moderation eliminates a lot of potential toxic material, like every other comment devolving into a shitty attempt at a joke or a personal attack.

And for me, personally, the biggest difference seems to be that on HN, there is a way higher prevalence of people trying to figure out what is right, instead of who is right. I am glad to be proven wrong in an argument here, because it gets me one step closer to finding the "most correct" answer. And the arguments themselves (usually) tend to be very polite. While on reddit, it seems like the vast majority (outside of some specialized and fairly niche subreddits) is just obsessed with proving that their answer is the "most correct" one, so they go all out on the means to reach that goal, which tends to any sort of arguments to become very uncivil very fast.

Don't get me wrong, there's a lot of terrible people there but they're an easy to ignore terrible. When you read something downright asinine there it's really easy to write it off as someone who's 14 and hasn't got the life experience to realize how crazy their opinions are. When you read something asinine here it's attached to a profile that says "senior dev at company X and maintainer of open source project Y" and seeing supposedly smart people be so stupid is a lot harder to rationalize.

1. I keep getting older, and Reddit doesn't just stay the same, they get younger. There are a lot of kids on there from age 10-16, and these kids don't just stick to Teenagers and Memes; they will actively comment and discuss politics, capitalism, society, etc.

2. Upvotes mean agree, downvotes mean disagree. In this way, any opinion with even a slight majority will always triumph. If I took 45 Redditors who were liberals and 35 Redditors who were conservative, and both used the upvote and downvote buttons on opinions, then every liberal comment would have +10 and every conservative comment would have -10, which amplifies the 10 person difference by 20. Every opinion you see is one that the majority of the user base agrees with or likes.

3. In both age and other demographics, redditors lean towards young, liberal males with low/no incomes. This causes a lot of bias against certain schools of thought, the role of government in our lives, and people who are on the opposite end of the spectrum - old, conservative (women?) people with high income. It's just a product of the site demographics.

4. The last point I'll make - since Reddit creates this echo chamber of a certain set of ideas and opinions, people who have differing opinions tend to stop posting those opinions or participating in discussion regarding those opinions. What you're left with are the people who are actively frustrated/righteous/feel like wasting time who are spouting against the grain opinions, making those opinions seem even worse to most Redditors than they already were perceived. (For example, if you were wanting to discuss how great you thought Donald Trump was on Reddit, you were most likely not looking for a productive and informative discussion)

I will say the Reddit community is extremely toxic. I left the site about 2-3 years ago when in my opinion it got real bad.