Personally, I believe that a game that is intended to be "moddable" should have the correct interfaces for that. D2 for example - simple text files you say ? Well, was the game made that way for purpose or why ? I mean - almost anyone can find out if there are "simple textfiles".
The makers of NWN made it *completely* the other way round : They didn't make it "moddable", but instead the editor is made "gameable" ! I mean that NWN is basically one big editor with modules, at least that's how I understood it.
These interfaces for enabling a game to be "moddable" must be invented early, that's why I believe that <img src="/ubbthreads/images/graemlins/div.gif" alt="" /> 2 could be a good canidate (or candidate ?) for that, rather than already existing games where the devs didn't have this in mind.