Downvotes are cancerous, imo. It encourages petty fighting (great for "engagement" metrics) and discourages good faith contributors.
Even upvotes can be an issue. Oh, you wrote your comment the day after the post went up? Too late! Nobody's looking anymore, no upvotes for you. No replies either. You feel your effort has reached nobody and you have wasted your time, so you stop bothering to put in effort.
I think the generality of it pushes topics into "old and inactive" way too fast.
Also, reddit's moderation is terrible. Just look at how long hate subreddits have tended to last even after they started hitting r/all regularly. People who are worth talking to don't tend to hang out anywhere that starts to feel like little nazi camp. Subreddits are separated to an extent, which does help, but that only goes so far.
I think the visual design also discourages longer "wall of text" responses in a way that traditional forums often don't. Lots of things (especially anything nuanced) can't be said in a tweet.
I do think the size that comes from centralization is part of it too. "Communities" feel less community, and posts/discussions are abandoned too fast and almost never revisited (exceptions are posts that come up as the #1 google search result for a tech problem).
The default and r/all subreddits are the worst by miles, though.
But city subreddits and any kind of recommendation thread have been much shittier lately because of the bots and the chatgpt copypasters.
Little forums also have a lot more character in their visual design than reddit, and they tend to allow more customization for individual users too. It's easier to recognize a person behind a regular commenter if they have a big pfp image or a goofy signature.