Posted August 02, 2010

I would state it as:
(a) no dependencies on external services
(b) no elevated privileges
Point (a) covers things like activation, master servers, 3rd party game servers. There is a grey area of "soft" dependencies, for example a world-wide high-score list (or achievements, as they like to call it nowadays ;) - if it's not an essential game play element, and the game can work without it, it may not be a problem.
Point (b) covers kernel hooks, drivers, system services, other intrusive system-wide modifications. It protects the system from crashes caused by 3rd party code bugs and incompatibilities, avoid conflicts between various kernel hooks and drivers, and makes the game more compatible with future kernels - and even with completely different kernels like Linux (Wine). Although game installers usually require elevated privileges, I do not count it if the installed game itself does not require elevated privileges.