This has been a huge frustration for me when it comes to bug tracking. I have multiple clients who just aren't technical people and when they see the wall of options that bugzilla and mantis offer it causes them to give up and email me directly. I even tried vbulletin for awhile, but that had its own problems.
GitHub (once you get past the signup) is perfect in this regard. Just open a ticket and then comment on it.
I wish GitHub had more in regards to bug tracking, for example being able to create states (open, in progress, closed) as well as labels.
These days I'm sure there are comparable solutions but having the issue system through github means one less service I need to track, which is also nice (client gets code, client has issue tracking, done).
With all that said I wouldn't use github issue tracking for complex multi-stage applications where the environment is an important factor in the problem.