What would have been better, not just for non-gamers but for the sake of a better movie, is if the storyline stayed closer to the actual storyline of the town, but not make that backstory the main story and do something similar to the Silent Hill 2 game.
I think the writers did a good job adapting the story of the first game into a coherent story, but their version makes Alessa the main (only?) reason why the town is screwed up. Now that Alessa/Sharon's "cured", the town may as well as go back to normal.
And it also, as said by too many people to count, the plot wasn't structured well.
(minor GAME SPOILER here) Alessa wasn't the reason the town was screwed; it was a demon god that the cult worshipped. The child Alessa/Sharon was supposed to give birth to this god, but that caused... problems.
The cool thing about Silent Hill 2 was that the story was independent of the backstory of the town. The main guy found clues as to why the town was the way it was, but the main story of finding his dead wife was still dominant. Such structure also meant that other people could be drawn to the town and have their own experiences (in sequels), regardless of Alessa or the cult. As such, the town would always be haunted regardless of Alessa.
I think the movie Silent Hill would have been better if it took that approach.
And, to be honest, as a HUGE Silent Hill fan, I love the concept and atmosphere of the games a lot more than the complete backstory (which is conflicting and even a little hokey).
Oh well, there's always the games (which I HIGHLY recommend to anyone).