Definitely true about most subs, but not all. Some, such as /r/askhistorians, are very strict about low effort posts so what you end up seeing (amongst a handful of deleted replies) are very well resourced to whatever the subject of the thread is. Of course, this requires an engaged moderating team which not all subs have. That being said, it still doesn't quite match the magic of stumbleupon and clicking a link to be shown a page matching your interests from obscure corner of the web.

To be honest, I don't know why a similar app or extension hasn't come up to replace what Stumble did. Surely there's advertising potential there (1 ad per every X clicks) and even a subscription option (remove ads or access to unlimited interest categories for $x dollars).

I feel like even with moderation, Reddit has a natural limitation by dint of being "adoration by upvote". You can't start a conversation outside of the boxes defined by the sub. If your post comes across as even slightly promoting of an undesired subject it mostly falls into the spam and downvote bucket, unless the sub is very specifically trying to include that, you have gained pre-approval, or you have manufactured some kind of storyline that loopholes both rules and human emotions. The average mod team is prone to abuse of power, so they also come down hard on anything potentially disruptive to the intended discourse. The incentives then move towards posting on Reddit in an intentionally deceptive "influencer" mode at all times - equal parts hype, pity, and outrage.

And there's both a reason for that being the case(nobody wants spam, and moderating can curate effectively in the best subs) and for it being harmful(community interaction ossifies into a familiar set of things that get upvotes, which subsequently pollutes every thread).