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. I had the opportunity to play the first hour of Mass Effect 2, to which I'll describe utterly and entirely. In doing so, I will spoil that entire hour, give my thoughts on 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.