It's pretty simple/minimalistic. Here's their summary...

    * Deal with your issues just like you deal with email (fast, JavaScript interface)
    * Create and apply labels to issues to assign to users or categorize
    * Drag and drop issues to prioritize them
    * Vote on issues that you want to see tackled
    * Search, sort, and filter
    * Close issues from commit messages
    * Keyboard shortcuts

...from their blog post about it: http://github.com/blog/411-github-issue-tracker . It's also got a screencast there. If you'd like to see what it looks like from a user's perspective, here's the issue tracker for the issue tracker: http://github.com/defunkt/github-issues/issues :P