Point is, they're two sides of the same coin. Nobody seems to look down on the former, but I think it's just as foolish to assert or guarantee that a movie will be good as it is to say the opposite.
Like Kahran said, there's reasonable, and then there's taking it too far. You can be reasonable with your expectations while still being optimistic or apprehensive (heaven forbid) about an upcoming movie. Throwing out guaranteed predictions is pretty unreasonable, there's no two ways about it. Anybody who has ever seen an outstanding trailer for an awful movie can attest to this. I'll go ahead and say that I see a lot more of the "This movie is gonna be awesome!!!" A lot more than I see the opposite, around here anyway (before anyone throws the IMDB trolls in my face as if that's an argument). Happens every time trailers, screenshots, and tv spots start dropping for an upcoming fan favorite. Yet the same people who fly off the handle in that manner will chastise someone who expresses even mild apprehension or disappointment (Jeez, stop being so whiny and negative; stop complaining! YOU HAVEN'T SEEN THE MOVIE YET OMG). Reading over the BvS threads is probably the best place to get an example of this right now, but it does indeed happen everywhere, every time. I think that what people need to realize is that we're all fans here, whether you've enjoyed a particular movie or not.
The fan community would do well to be a lot more tolerant of others, but it seems like there's a large contingent of fans who are more content to have an exclusionary circle-jerk than anything else. Negativity that comes from a good, earnest place gets far more scrutiny than the zealous, dogmatic worshipping displayed by some fans around here. The sad part is that these folks are the ones who scream the loudest about wanting their opinions respected and their voices heard, yet they do everything they can to discredit and silence people who feel differently. I'm reminded of when someone created a thread complaining about the perceived bias of critics against this franchise...after only 50 or so reviews had been posted. That's just sick.
Even in spite of how I feel, I'd still say that this is a pretty good community, all things considered. Be that as it may, this place has a long way to go.