Point # 1: they WILL have some anorakism. luckily, this lets you introduce advanced rules fairly quickly. one thing to mention though is for heroic feats: make sure all parties understand what will happen should the hero succeed OR fail, BEFORE any dice are rolled. remember: these guys play to win.
Point #2: this you'll have to mediate with your fellow gamers as well. will they want the more serious political-strife based background, or is "lol kill it" fine with them? you should go with the annihilation method if at all possible, but some of the other examples include: capture the flag, capture the briefcase, defend for X number of turns, retrieve the treasure (this one's fun. get a bunch of treasure chests, fill one with the super-secret-object, and then mix and match em until nobody knows what's in what case, and spread across the map), or of course, Road Warrior.
Point #3: do the shopkeepers know you well? like on a first name basis? if so, just bring up that you found a d6 based wargaming system that uses legos. when they go "4 srs?", hook em with brikwars.