Most previews tell you about the game as a whole, with limited information so nothing's spoiled. They describe gameplay mechanics, graphics, characters, essentially everything you see in a demo or trailer, but in greater detail. It's a look at what game developers and publishers want the consumer to see, not necessarily what ends up being sold on the market.
This isn't going to be a typical preview. With the opportunity to play the first hour of Mass Effect 2, which is described utterly and entirely. In doing so, the entire first hour will be spoiled, this is a preview of everything, from gameplay to storyline and dialog to weapons. If you don't want Mass Effect 2 spoiled for you, don't bother reading this article. If, however, you want to know more about 2010's first blockbuster RPG, read on at your own discretion.