I really liked the ability to comment on individual cells in old Observable, and I think the new notebook comments are not as useful.
My main use case is for education, where I have students fork an “assignment” notebook with multiple questions in it - each in a separate cell (often with other cells demonstrating concepts, etc… along the way). The students then answer the questions by adding cells with their answers (which might be text, code, etc…). They then shared their notebook back with me when they are done. I then marked the notebook by commenting on their answer to each question.
Yes, I can just type all of the feedback in the comments at the bottom of the page. But this is clunky for marking, because would I have to constantly jump back and forth between the answer and my comment, and it is also clunky for the student to read, because they will need to jump back and forth as well.
More generally, I find that comments on a document are most useful when they are close to the specific content they are commenting on. For example, the editing comments in MS Word, or the discussion comments in Perusall.
Please bring back contextual comments in Observable.