Simplicity is the key to "Get in and Go" gameplay which breeds a larger fanbase.
The original Team Fortress scared people away with it's multitudes of tricks, specialized weapons, and odd gameplay mechanics (So I'm infected with what? who? I don-*DEAD*).
TF2 is by no means an overly complex game, but it is one of the most enjoyable Source games ever to come out of the Valve Studios due, in my opinion, to it's simplistic system that appeals to more casual gamers who want to just play for a bit as well as appealing to the hardcore gaming crew by having very polished, top notch gameplay overall.
-Then we get into the whole casual vs hardcore argument thing and... let's just not get into that.
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On a more SCOUTUPDATEzOMG note, "SCOUT UPDATE zOMG!!!"
I'm excited to see what they do with the scouts pea shooter (the pistol) and I hope, no matter what it is, it comes with more than a handful ammo...