bit-tech's David Hing has penned a short editorial examining the demo version of Firaxis' XCOM: Enemy Unknown. Although I personally wasn't impressed by the demo, the author seems to think that Firaxis did a good job in communicating certain aspects of the game.
It would have been really easy to just showcase the turn based meat of XCOM and many would have been perfectly satisfied with this. It would have given a taste of the core gameplay, but it would have been ignoring what holds it all together.
You get some options about where to set up your base and are given a quick whistle-stop tour of the facility with a few decisions required in terms of research and engineering before being taken away to the next mission where you are let off the hook a bit more and not driven off a cliff by the tutorial.
Once you play the first game, it actually takes quite a long time before you're left with the keys to the base and that particular drip feed could have been sped up slightly, but as a taste, this first glimpse is just enough to get you interested.