My main complaint with Reddit since the early ~2010s has been the crap post/comment quality. In 2008/9 when I first joined, the comment quality was very high. Then memes, puns, one-liners, and hivemind took over and ruined the site.
An ideal Reddit would be able to show you posts/comments that you're most likely to enjoy, such as via a "sort by" option tailored to you, or other sort by options like an "insightful" that actually works and doesn't just give you low effort one-liners that add nothing to your life.
High quality content in general needs to be better incentivized, and right now it's not. Anyways I have a lot of ideas around that I wrote elsewhere so will refrain from dumping then all here [1]
The challenge with developing a Reddit alternative however unfortunately is not technical, it's overcoming the chicken and egg problem where nobody cares about your site if no one else is on there.
[1] https://jsavage.xyz/2022/03/13/the-downfall-of-reddit-why-re...