First of all, if the film's ending came out of "left field," then I wouldn't have figured it out thirty seconds in like I did. The film opens with a shot of a tombstone with some dates on it, and knowing going into the film like I did that it was set in a village in the middle of the woods with no contact with the outside world, I wondered, "How do they know that they have that date right if they're not in contact with the rest of the world? I bet that it's actually much later than that." Sure enough, that was the ending twist - it's actually present day, not the nineteenth century. It doesn't seem too unreasonable to expect that people buy this ending as believable and led up to sufficiently.
The other problem people have with the movie is that Shyamalan reveals what should have been the major twist -- the creatures that live in the woods and keep the people of the village afraid of trying to leave are actually the people who started the village, dressed up in elaborate costumes -- too early, partway through the movie instead of at the end. I know exactly why he did this. People go into his movies now looking for the twist ending and trying to guess what it's going to be. This was his attempt to get the audience out of that mindset early on, by making them think they'd already gotten the twist ending, leaving them genuinely suprised by the real twist ending at the very end of the movie. I think it was the smart thing for him to try, but it threw the audience off so much that most left the movie unsatisfied, feeling manipulated.