They've been pushing hard for growth as well, which I think is probably the single most devastating thing to an online community. There is some amount of growth that can be tolerated while maintaining a sense of community and common culture, but when the new members are pouring in by the thousands every day, that is no longer possible. Most participants will be strangers to each other.
What few subreddits have survived this are either incredibly niche, or aggressively working to make new members lurk before they post.
I think "common culture" is precisely the problem with reddit. Before reddit there were many different separate forums. Each had to be signed up to separately and each had their own culture. With reddit there's little barrier between reddits, which has lead to a shared culture across the whole site. The culture is generally friendly, but is immature and superficial. It's great for small-talk, but useless and frustrating if you actually want to learn something.
Maybe I'm bitter, but I used to really enjoy r/rust up until 2 years ago or so. Now it's completely inane and it's no longer worth the time to sort through the noise to find the signal.
Right, when I say common culture I mean culture common to a specific subreddit. I don't think reddit as a whole has a common culture. It's like a train station and not like a village, everyone is always passing through.
It's basically the difference between a village and a city. Reddit used to be like a bunch of villages, now it's like apartment buildings in the same city. Everyone is always a stranger in a city, people are always coming and going. You can walk down the same street the same time every day and see new faces every time.
That is exactly the same dynamic as is present on most of Reddit. Almost everyone you see is a stranger. Used to be (especially on old forums) you had some sort of relationship to the people you interacted with, you had a feel for their personality. Now it's just yet-another-new-username.
I think this is a major part of why there is so much tribalism online lately. People are desperately latching onto any sense of common identity to have something in common with the strangers they keep meeting.
What few subreddits have survived this are either incredibly niche, or aggressively working to make new members lurk before they post.