I've been using the site for ~14 years and watching its decline has been quite difficult. I really like the premise, but it clearly hit the limits of its system many years ago. I recall reading some commentary from one of the founders near the beginning. They were working on research to exploit crowd-funded effort. Effectively incentivising users just enough to encourage them to work for free. They came up with the distributed moderation system. Give egoists just a little bit of power and they'll spend 12 hours a day moderating Reddit for free.
This worked fine with smaller communities and the ability for competition when moderators overstep their mandate and make their communities a bad place. Unfortunately, for many reasons, this is all failing.
1. Reddit management has made their political leanings clear: if you are conservative, you are unwelcome. When I say "conservative" I'm not referring to the far-right. I'm referring to mainstream conservatism about family values, hard work, religion, etc. No example could better encapsulate their position than their "Anti-Evil Operations" team, which bans people for any and all reasons, including unpalatable political positions. Admins/staff on this team have been quite candid about their desire to ban conservative views. This aligns well with Reddit's announced IPO. Sanitising discussion to be advertiser friendly will help their bottom line.
2. The user base leans heavily left. It seems certain large websites have a theme now. When one considers the kinds of discussion which happen on Facebook, Twitter, and YouTube, we all know which way those sites lean. Reddit is pretty hard left now. For example, the /r/Science subreddit is 24/7 "Study finds Trump voters have low IQ." The Joe Rogan fun subreddit is 24/7 "Does anyone else think Joe Rogan is Hitler?" There is zero space for nuanced discussion except in some very niche subreddits, and they are all inevitably taken over by a power mod who purges nuanced discussion.
3. The anonymous mod system is just not equipped to deal with tens of millions of users in a single subreddit. The top mod is the dictator, and they always end up enforcing their biases on the sub; overtly and subtly. This is exacerbated because of...
4. Subreddit squatting. A perfect example of this are the country name subreddits. Everyone from said country new to Reddit subscribes to these subs. They are all cheerleaders for a particular political party within each country. If you don't like that political party, you're marginalised and usually banned. It's not as simple as creating a competing country subreddit because few people will ever find it. No one thinks to search for "TrueCanada" if they're looking for "Canada." Of course, the the original subreddit automatically removes any mention of the competing sub, and bans anyone discussing it.
There are just too many issues to discuss them all, or in much detail. IMHO, Reddit is ready for a Digg-style exodus. People are just waiting for a clear alternative.