Right now it's possible to mark whole pull request as "needs work", but it's not very practical.

Often a lot of time passes between review and the fix and reviewers just forget which issues should be solved before the pull request can be considered approved. Reviewers need a way to see all issues they are considering as blocking.

Reviewer can use tasks to mark blocking issues, but authors can mark the tasks as done themselves. We need something like tasks, but the tasks that only reviewer can mark as "done".

My suggestion is to add a checkbox to a comment box with a label "needs to be resolved". Having comments like that should block merging until the reviewer marks the issue as resolved (resolution can be an actual fix, or just a talk that convinces reviewer that no fix needed). And reviewer should be able to see a list with all comments marked as this.

One of the options is to mark the whole pull request as "Needs work" when such comment exists, but please note that "Needs work" is while much better than "Reject", is still a slightly irritating name for a button, and when doing pull request it's very important to keep level of stress as low as possible.

Most often than not we don't really need to mark a PR as "Needs work", we just need an answer from the author. And, before that answer is received, we want to block the pull request from being merged. It's not like it "Needs work", it more like it "Needs resolution" or something. Right now reviewers tend to not use that button because of that. Please consider to rename it to something even less disturbing.