I must admit I didn't think much of Kristen Stewart's acting ability in all of the Twilight movies (though I'm still a fan of the movies). She always seems rather one-dimensional in her emotion and approach. In this movie when she tried to play something other than the victim (in other words, the tough warrior) it just didn't ring true. As a result the final scenes of the movie which should have been climatic and powerful were weak and never really gave the viewer the "WOW" factor these scenes should have shown.
Oh and whoever devised those shredded metal warriors in the end should be fired.... Just didn't really work either.