If you use Github a lot for browsing code, then you will find it irritating that it does not have a project explorer. That is, you want to explore the directory structure of a project and view the source code of the files by clicking through the directory structure. This is a standard feature in all IDEs and most editors.
On any Github project page, it gives a sidebar that shows the directory structure and you can click through to view the source code of any file in the project.
Tried with: Octotree 1.3.0, Chrome 35.0.1916.153 and Ubuntu 14.04