An entire mod developed in a week. Is it possible? We just proved it was. An enjoyable mix of ego-shooters, fast-based CTF gameplay and team base attack/defend. The progress is available on the Facepunch thread.
Another hasted model done with grace by Ihazard (Facepunch Forum Username), this time in the form of a playermodel concept.
Add Shaders, Light Maps, and Bump Maps, as well as higher quality textures. It doesn't have to be Valve Quality, but enough to make it look decent.