To me, Reddit's the last bastion to find good conversation for people with many niche interests without going too crazy. (4chan is a bit too eccentric for my tastes)
I frequent the soccer, manga, headphones, metal, male fashion, thrifting, cooking and fitness subreddits.
No other forum let's me feel like a part of such a wide array of topics with discussion being the core focus. To be able to maintain anonymity while nuturing a community is incredibly hard, and Reddit had somehow managed it. The communities in Reddit are second to none. From meta jokes to entire sagas, Reddit in some ways reminded me of the times when I played Mmorpgs and joined guilds.
The removal of custom subreddit themes was where to me it started going wrong. I didn't care what they did with the popular subreddits and r/all, as long as they left the niche subreddits alone. But no, they had to interfere.
Reddit's forced UI change is to me a symptom of a greater problem. The idea that a company never stops growing. Sometimes you hit gold and the best decision is to stick with it.
I don't think any website offers the product Reddit does, and allows easily migration for entire communities without compromises. Thus, if the current web landscape stays the same, Reddit isn't going anywhere. But, it took weeks for Digg to die once Reddit came alive, and the introduction of a well moderated website for discussion may be all it takes for the Reddit exodus.
It is no surprise, that I find myself on HN typing this comment. An HN clone may just be where Reddit refugees find their calling.