My greatest complaint is that it wasn't more epic. At only 90 minutes, it felt like they missed an opportunity to really flesh out the characters (something that was sorely needed), steadily increase the tension and sense of overwhelming dread, and just generally add more meat to the action.
Also, the fighting (particularly the first major battle) seemed poorly shot and/or edited. I really, really, really hate how so many directors these days feel compelled to push in on the action, getting so close that you can't really tell what the hell is going on. And then they compound that error by cutting things so quickly, which, I understand is meant to convey the frantic, chaotic feel of actual combat but it ultimately just leaves me feeling lost and disconnected as I lose all sense of the battle. I found that really frustrating.
And, sadly, the ending felt somewhat anticlimactic and more than a little awkward. I wish it had been handled differently.
I think an additional 30-40 minutes of quality writing/footage, if they'd made a greater effort to develop the characters and build some sort of emotional connection between them and the audience, if they'd done more with the action, and if they'd found a better way of bringing things to a close, it would have made the movie something really special. As it is...on a scale of 1-10, it's somewhere around a 6 or so. Worth a watch, but a little disappointing when you realize there's a lot of wasted potential and it could have been a much better, more satisfying on every level movie.