When I'm posting a comment which needs the attention of someone not already in the watcher list, I have to stop what I'm doing, go to the watcher list page, add that person, come back and resume posting the comment.

This is very irritating and happens to me a lot of times in the day.

Probably you can put the "Add Watchers" widget along with the comment form so that if I want to add a new person, I can do that from there itself.