Popular issues get a ton of comments on them containing simply:

+1

In this instance, all the existing watchers on an issue get notified, followed by the obligatory comment from someone else:

Please stop adding +1 - if you want to show your support for this issue add yourself as a watcher! You're doing nothing but annoying every other watcher of this issue.

To save everyone the bother, and to help actually reflect the intent of the "+1" comments, I propose comments containing only "+1" are converted to watchers rather than being added as a comment.

An example of a popular issue with this occurring in it:

https://bitbucket.org/site/master/issue/3288/remote-pull-requests-to-google-code-and