In my experience it's the instability (YMMV on that, for me in manifested in videos not working more than they did, heavy stuttering while scrolling. Not issues I have in RIF), low content density, significant number of trackers. Obviously I don't like all the ads being shoved in the middle of my feed, but I'm not against monetization.
I mean inlined ads for one thing - we're talking paid marketing posts masquerading as user-created content, some of which are utterly obnoxious (e.g. the 'he gets us' jesus ads)
Same as the redesign, it's optimized for consuming images in a platform I came to to read discussions. And it does a horrible job facilitating text for long form discussion. It's trying to be instagram, I don't want instagram.
Also, I don't think it's at all controversial to suggest the video player is one of the worst I've used. I dread trying to load up native videos on desktop (new or old design), I can't imagine it being better on the app.
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those are the biggest issues that will never be reconciled. Others include features you probably don't care about unless you're a power user. Ability to filter posts/users. favoriting certain subs, various mod tools that STILL have no alternative (even on desktop), proper responsive design for tablets, an actual other discussions option, visible flairs etc. I could go on all day with nitpicks (each of which I requested to reddit at some point years ago. Some of which were promised but never realized).
