Villains can have all kinds of crazy, murderous, evil powers and no one really cares too much about the source or the fact that the power appears limitless, but if the hero pulls something similarly amazing / improbable out of his bag of tricks to eventually win, people are very quick to call bullshit.
That's an interesting point. I haven't thought much about this discrepancy before.
In this case one can argue that since the monster was using the imagination of the children against them, the children could likewise use this power against the monster, thus doing very improbable things. I think King made this argument, though I don't remember the details.
If you provide zero foreshadowing that there's any sort of weakness for the villain or strength for the hero that can potentially turn the tide, it's going to look like you just forced a heroic ending for its own sake. But even when it's executed well enough, a lot of horror fans just don't like accepting that big bad villains can be vanquished at all, ignoring the idea that there's a balance to damn near anything in nature and likely supernature as well.
Just as an anecdote, I'm watching Kara no Kyoukai, where a premise is that everything has a weakness. There is even a girl who claims that if she met God, she could kill him. In this case those things are pretty much foreshadowed, and they were in IT too.
