Everywhere in Launchpad, a green text link with a (+) icon behaves like this: if you click it, Launchpad will ask you for more information, and then add something.
Everywhere, that is, except two places. One is the "Add comment" link, being fixed in bug 408765. The other, which was my fault to begin with, is the "Subscribe" link: this subscribes you to a bug report without any further interaction, notably unlike the "Subscribe someone else" link immediately below it.
This bug will be fixed when subscribing yourself to a bug report uses the (+) icon consistently with its other uses in Launchpad.
A banal way of fixing this would be for the "Subscribe" link to ask for further input, namely whether you're sure you want to subscribe.
Probably a better way to fix this would be to stop using the (+) icon for subscribing altogether. Subscribing to a bug report is only "adding" something in a rather abstract databasey sense. It might make more sense to use an instant-apply checkbox for subscribing/unsubscribing yourself, and a different icon (or, if the form opens inline, an expander triangle) for the link to subscribe someone else. Non-feed subscription actions on other Web sites typically use an icon of an envelope, an arrow, or occasionally a newspaper or a pen and paper <http://www.google.com/images?q=subscribe&imgsz=icon>.
Since the appearance of the Subscribe function affects the appearance of the Unsubscribe function, it may make sense to design the solution to bug 401658 at the same time as this bug.
I agree that "Subscribe" is the one action that seems different than all others because it happens immediately. I'm not sure that adding an intermediary "Are you sure?" overlay is an improvement. In fact, it could annoy people. I actually like that subscribing happens quickly on click, as it's something I do frequently, and I don't personally have a strong association with an icon to a particular action. But I also have no problem changing to a different icon, should the design gurus deem it so.
I'm marking this low priority given all the other UI work that needs to happen, and leaving it to the design gurus to discuss.