What's the difference between saying they suck before release and deciding it sucks when you see it in the store and grab something else instead? I can't think of a recent example, but I won't be buying Dead Space 2. I know no more about the game than when it was first announced. All I know is, I didn't enjoy Dead Space. Extrapolating from that, Dead Space 2 sucks, for me. Similarly, if I didn't like MW2, it's a fair assumption I won't like MW3. Same goes for BF3, Arkham City, Saints Row 3, Skyrim, Skyward Sword, GTA V, Borderlands 2 and whatever the fuck else is coming out soonish. People usually have prior knowledge or experience of a game, even if it's just playing an older installment.
Yes, some people just say something will suck to troll, or because general consensus is it won't. But don't just call everyone who forms an opinion on a game without playing it an idiot, because we do that every time we make a decision to buy one game over another.