[onion2k]

it would just require that common web platforms allowed their current comment systems to attach comments to paragraphs in the article, and show these comments as side notes

I think that's a somewhat view of what a user contribution could look like. Adding feedback is great, but imagine what the web could be if users could do more than just comment. I'd love to see a blog application that supported user contributions like fixing spelling/grammar, adding links, injecting additional paragraphs to explain complex topics, captioning pictures, etc. All those things could be suggested as comments that the author would manually use to improve their article, but I think it'd be better (faster at least) to do it automatically.

Idealistically I'm thinking of something that's the best parts of Medium and Wikipedia.

[TuringTest]

How is that different from a wiki? :-)

Mediawiki, the software behind Wikipedia, already has all those features, includeming the possibility of moderation by privileged users or automatic updates, on a per-page basis. You'd just need to use the software for blog content rather than encyclopedic content.

[onion2k]

How is that different from a wiki?

Every app is a CRUD app if you ignore enough detail.

[TuringTest]

I'm asking you about the details that make them different, and which you consider relevant for a "web with user contributions" in the way you described.

[onion2k]

The difference is that those features would be implemented differently in a blogging context. I think that they're things that would improve a collaboration on blog to move from simply getting feedback from users as comments for the author to use to actually collaborating on something to make it better. I haven't thought about what those implementation differences would be; it'd be a lot of work.