Let me start by saying I didn't hate Snow White and the Huntsman. It was exactly what I expect from most movies nowadays- a light, fun film with cool special effects, entertaining fight scenes, some funny lines, and a predictable story with some obvious flaws. The bar is clearly lower for movies than books, and I think writers can learn from some of Snow White's shortcomings. I should note that problems in a movie's story line are not always the fault of the writers. You never know what wound up on the cutting room floor that could have addressed these concerns. With that in mind, here we go.
Warning! Spoilers ahead!
1- If a character is going to make a stupid decision, give us a reason, even a lame one.
This movie could not have happened if the queen had just killed Snow White like she did the king and pretty much everyone else. A couple of potential excuses spring to mind here:
-"Royal blood is especially powerful but the magic won't work unless I wait until she comes of age to suck the beauty/life out of her..."
-"She reminds me of myself at that age and I can't bring myself to kill her."
(I think there's something to the latter one, but it's never spelled out clearly, it doesn't match with the queen's other actions, and we don't get a flashback to her "origin story" until late in the movie. By then, I've spent over an hour thinking how dumb the queen is.)
2- Time frames should make sense and events should fit together properly.
Snow White escapes into the forest and must reach the duke's castle which, we later discover, is about a two day trek. The bad guys try to catch her, fail (partly because their horses can't cross the swamp at the edge of the forest, come back with the huntsman, fail again, and head back into town. Meanwhile, word has somehow reached the duke that Snow White is alive. He passes this information along to his son who immediately takes off and manages to get to the town outside the evil queen's castle ahead of the bad guys (or at least in time to join them on their hunt- it's early on the same day).
Snow White and the huntsman journey all day and are boated across a lake to a village. A few hours later, the bad guys attack. This is weird, because Snow White, guided by the huntsman, took off as soon as the bad guys failed to capture her the second time. Yet the bad guys managed to: go back to town; get a couple new recruits; get their horses through the previously impenetrable swamp; track Snow White through the forest without the aid of the huntsman, whom they supposedly needed only hours before; either get their horses across the lake or ride all the way around it; and still arrive shortly after Snow White.
Snow White escapes and makes it to the duke's castle. She immediately rallies the troops who (riding horses burdened by barding and armored men) make it to the queen's castle in a few hours. It might be that the duke's castle is only a few hours ride around the outskirts of the forest from the queen's castle, but that leads us to another issue:
3- If a character has awesome powers, yet does not use them to their full or logical extent, we need to know why.
It's established very early in the movie that the queen can't be killed (at least, not by anyone not named Snow White). She can also make warriors made out of... I don't know... shards of mirrored glass, which our heroes can't manage to take down until Snow White kills the queen. She also has a regular army. Considering all this, why has she let the duke sit there in his castle only a few hours away and oppose her? Why not march over, let the mirror warriors do their thing, and wipe out all the men and ugly women (she needs the young, beautiful ones for her magic), knowing you can't be harmed?
Along those same lines, we have a scene late in the movie where the queen is straddling Snow White, knife raised, ready to cut out her heart, when the huntsman and the duke's son arrive. Does our invincible queen who can't be killed by anyone but Snow White spare a few moments to finish the job? No, she turns into a flock of ravens and flies away, leaving her only threat alive.
This leads us to...
4. Magic should make sense.
Can the queen be killed by a "regular guy" or not? At the end of this scene we see her back at her castle, clearly weakened. Shortly thereafter, she's all juiced up again, standing among bodies of young women. The implication is that she was vulnerable in her weakened state, and thus needed to flee the huntsman and duke's son. But wait a minute! That would mean Snow White isn't the only one who can kill her. Deny her a supply of beautiful young women and wait for her to weaken, then take her down. So... is Snow White the only one with the power to kill her or not? The average moviegoer probably didn't give this any thought, but I think the average reader would call you on it in a book.
Other magical issues-
-Why did the huntsman's kiss wake her up but not that of the duke's son? Is it because he shed a tear, or is it simply because he's Thor?
-Snow White can kill the queen because of her pure blood, so why don't we need a little bit of Snow White's blood on the blade before she stabs the Queen? Blood magic is pivotal in the movie and it would be easy to accomplish in the climactic scene. (Or did they do just that and I missed it?)
Speaking of the climactic scene...
5. Fight/Battle scenes should make sense.
I need to attack a well-defended castle. Do I make a plan of attack? Build siege engines? Nope. A cute girl makes a speech, we ride off in a frenzy, arrive at the castle, pull up short, and all together give a collective, "Hmmmm..."
I'm defending a castle and I've got plenty of boiling pitch to dump down onto the attackers. Do I do it while they are milling around waiting for the portcullis to be raised? No, I wait til it's open and the invaders are charging in, then I dump it onto a few of the stragglers.
I'm Snow White and I'm the only one who can kill the queen. Everyone else is a diversion. Do I disguise myself, join the dwarves, and sneak in through the sewers while my army draws the attention of the defenders? No, I plant myself right in the middle of the vanguard and right through fireballs, arrows, and pitch, knowing all the while the dwarves might not manage to even get the portcullis raised, thus leaving me outside where I can't get through to the queen. When I finally get inside, I fight my way right through the middle of the defenders to get to her.
And while we're discussing fighting, the sewer, the portcullis, and all that good stuff...
6. Details matter
The final battle calls attention to one of the strangest details of the queen's castle. We know from Snow White's initial escape through the sewers that the courtyard is a good forty or more feet above water level. Yet, when the attackers get through the portcullis, which is down on the beach, they immediately pour into the courtyard. It apparently exists simultaneously on two planes. Also, if your portcullis and courtyard are down at beach level, what happens when the tide comes in? This would have been an easy fix- just show the troops charging up a ramp to a portcullis that sits high above water level. End of problem.
Speaking of easy fixes, there's a scene in which a dwarf is shot and the duke's son returns fire. His arrow strikes the big bad guy high on the left shoulder at his collarbone. The wound is instantly fatal (he's no Boromir). How hard would it have been to have the arrow strike him in the heart? Meanwhile, the dwarf who has been skewered (and I mean in one side, through the heart, and out the other) lives long enough to speak words of encouragement and put one of those very Bella expressions on Snow White's face.
While we're on the subject of Snow White, how is it that she's been imprisoned for a decade and believed dead, but some random guard immediately recognizes her when she escapes? How does anyone know what she looks like now? And how is it that a girl who's been locked in a small cell for all these years has the strength and stamina to make a two-day journey through the forest, then strap on armor, grab a sword, and give as good as she gets in battle?
And why didn't the bad guys put bars across the sewer mouth after Snow White escaped through it? We know from the dwarves' trek through it that some sections of it were barred. Why not that one?
I could go on, but you get the picture. Overall, these issues probably did not bother the average moviegoer at all, and maybe I'm off-base with a few of them. That doesn't change the fact that they were all things that distracted me. A writer doesn't want to jolt her or his readers out of the narrative by making mistakes such as these. If the reader stops thinking about the story and starts thinking, "This doesn't make sense," then we haven't done our jobs well.