I frequently subscribe to issues on GitHub, be it bugs I want to see fixed or features I would like to see implemented. Then every once in a short while I get an email notification about one of those obnoxious "me too" or "+1" comments, by which I mean terse comments with little to no content other than "me too" or "+1" or some other variant bearing the same meaning.
Me-too comments under bug reports are the most untolerable. If you have more details regarding the issue (e.g., a more reliable reproducer) or insights into what's really going on, then by any means post them. On the other hand, if you can't provide anything helpful, then just keep your mouth shut, and quietly press "subscribe" if you would like to be kept posted. Posting a me-too comment adds nothing to the discussion, does not expedite the resolution a tiny bit, and only serves to annoy all parties involved.1 As always, submit a patch if you're dissatisfied with the progress. Keep in mind that no one is obligated to fix bugs for you in FOSS.2
Me-too comments under feature requests are more understandable, though I genuinely doubt that two or three people requesting a feature instead of one would make a big difference. After all, the issue tracker is not a feature voting platform; most folks understand this and behave themselves, so "me-too demand" isn't even remotely accurate at reflecting demand.
Me-too folks: please stop being childish. If you have nothing to add, don't add anything (unless otherwise requested).
01/20/2015 Update. I came accross dear-githuub/dear-github just now, which was started a mere six days ago, and the open letter of which also places +1 comments on its list of biggest problems on GitHub.
03/10/2015 Update. GitHub is finally reacting. See Add Reactions to Pull Requests, Issues, and Comments.