reviews may benefit from a concept of responsibility/assignment

Most code reviews that don't immediately complete seem to have the ball in somebody's court to develop the code more. It may be the submitter's responsibility to update the patch, or the patch pilot's responsibility to polish and land it, or someone's responsibility to review it. If that particular person isn't responding there are things other people can do, like ping them, or do it themselves. But in the normal case, that review should be high priority for the responsible person and low for others.

At the moment determining "who can move this forward" tends to require reading the whole thread.

It might be useful if there was an assignee, as for bugs.