The things that make a plug in great? Well, a lot of things contribute to the greatness of a plug. I think it needs to have a well thought out, clear, interesting, and fun plot, that also has sub-plots built in. It also needs to have plenty of missions, a large variety of missions (just to add flavor), plenty of factions and governments to play around with, and added small little tidbits that make you want to play it over and over again. The graphics must be great, and the addiction level at maximum. If you have all of things, or most of them, you have got a good plug in on your hands.
-Wire
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He would, after all, be an ambassador; between the old and the new-between the creatures of carbon and the creatures of metal who must one day supersede them.
-a quote from Arthur C Clarke's short story, "A Meeting With Medusa"