For the full tour, check out the GitHub blog post about Issues.
Issues is unique in that there aren't any Owners, Priorities, or Milestones - only labels, like GMail. This allows project owners and users make up their own system to suit the project. You can also sort the issues by drag and drop, so you can move the important ones to the top.
Try out GitHub Issues, and let me know what you think. Disclaimer (yesss, I finally get to make a disclaimer): I'm an advisor to GitHub.