Hi, Peter, hope you don't mind me posting here. Something that jumped out at me on first glance was that the phrase “multi-line comments” could be taken to mean source-code comments rather than talk posts/replies. “Talk page comments” is unambiguous. Perhaps prefer “multi-line list items”, since the new markup would apply to all lists, not just talk pages?
Topic on User talk:PPelberg (WMF)/sandbox
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I agree that this could be confusing. OTOH, most editors don't think that they're making a list when they type :
to "indent" a comment.