When I started on Reddit there was already an element of things having changed, a move away from niche technology discussion towards lowest common denominator stuff and discussion by meme.
In general this was fine as you could just move to smaller subreddits, and the bigger pages had fun cultures that permeated the others and gave it all a sense of unity and shared interest even when you were in your bubble.
The difficult change I've seen in the past few years is the rising number of hate and harassment subs, and Reddit tolerating them. If you scroll through the top of /r/BanFemaleHateSubs or /r/AgainstDegenerateSubs you'll see how bad things have become and how unsafe the general environment is.
It's not that discussion has become worse in an Eternal September way, it's that discussion has become openly threatening, with innocent women having their pictures and social media links shared for strangers to discuss rape and murder fantasy, and this gets 10s of 1000s of subscribers and Reddit routinely ignores reports about it.