The ability to play quest games offline without having to actually install anything, but just run a program from a memory stick or CD.

This would mean that it would have to be a Portable Application, so it would not do anything to the settings on the host computer, and save everything in files in its own directory, instead of somewhere like the registry, or Program Files.

I have a friend who uses a computer as a guest, and isn't allowed to install anything, so to play he has to use Textadventures.co.uk, which isn't always preferable.