"Nested comments sorted by upvotes" is, for free and frank discussion, inherently far worse than non-nested in-line comments. With the latter there's no hive-mind effect, no consensus-seeking, no dopamine/approval-chasing. Also, traditional forums tended to encourage longer-form posts (which you can still see in places like Spacebattles), which naturally contained quite a lot of technical detail and pictures, whereas Reddit (and HN) are optimized for very short comments. In Reddit's case, smarmy one-liners, usually.
But the main problem, to repeat for emphasis, is that the upvote/downvote system (even if it's fair and used virtuously, and it usually isn't,) stifles disagreement and debate.
When I append "reddit" to my google search query, I'm not looking for "disagreement and debate". I'm looking for specific information on non-political topics, such as repairing my car, finding a good product in the sea of garbage, or learning new techniques. Such topics are typically discussed cooperatively rather than adversarially. For this stuff, consensus-seeking is a feature not a bug, and where the consensus appears inadequate, I'm well capable of looking past the top post. Reddit's format is not perfect, but it's better than having to read through a 30-page thread in which most messages are irrelevant to most other messages. Such threads are linear only artificially through a UI that hides the structure of the underlying conversations.
If you don't like the upvotes aspect of reddit, we could settle on the same nested format but without sorting by upvotes. But with forums, we don't even have that.
Reddit's comments aren't one-liners because Reddit's format encourages that, it's because it's the most popular site where everyone goes. If forums were as widely popular, they would see the same people making the same comments there too.
But the main problem, to repeat for emphasis, is that the upvote/downvote system (even if it's fair and used virtuously, and it usually isn't,) stifles disagreement and debate.