Trailers can be a huge tease with no payoff. Like the original MotorStorm trailer. When the public finally admitted to itself that it was little more than a hoax, the fact that the final product matched the prerenders was not just technically incredible, but a masterpiece of gambling. Valve's David Speyrer talked about how difficult it is to promote a product that hasn't been finished yet.
We're going to try and do something pretty ambitious for that project. We don't want to overcommit. If you look at the Episode Two trailer that we shipped with Episode One there's some pretty radical difference between what you see there and see in finished game. That's really an artifact of making a trailer for a product that's still in heavy production. You just don't know where you're going to end up.
No no, Valve! Overcommit! And then work crazy long hours to deliver the promise, abandoning friends and family to do so. And you know your children? Be honest, they were a mistake anyway. RPS Interview: Episode Two's David Speyrer [rockpapershotgun]