I like it!
I've done the following enhancements (http://userscripts.org/scripts/show/51245):

1. Add tooltips to show the current list of watchers: to "Watchers: n", the new icon (form_blue_small), and the "Watching" action

2. Make a transparent submit to ManageWatchers.jspa and stay on the issue page.

Other possible improvements:

3. Option to use the list of assignable users for the current project, instead of a preset list: useful for internal projects!

4. Filter the "add watchers" list to exclude current watchers

5. Removing watchers