Go get KOTOR now

The difficulty in explaining the potential for a game like Mass Effect is magnified if your intended convert hasn't played the earlier titles in what I refer to as "The Holy Games." Knights of the Old Republic is one of the most engaging, well-written RPGs you'll ever play. And since RPGs require a level of writing at least half a cut above other genres, it means it's light years beyond something like Gears of War.
So, make sure your convert has played something else from BioWare first. The Mass Effect conversion will take place in time.