Prisoners is a largely generic revenge / suspense / mystery "they have my kid" tale with a predictable plot that gives itself away too early. No attempt at realism, this is Scooby-Doo style investigation - lie detectors are treated seriously, "bad guys" are largely 1-dimensional with poorly explained motives designed to surprise rather than give insight, etc.
It tries to make up for this in a few ways: a) bump up the intensity of scenes (comes across forced and awkward), b) add extra violence and dark semi-religious symbolism (seems out of place and largely meaningless), and most importantly c) maintain an air of mystery by throwing left-field plot points at you every 3-5 minutes to keep your attention. 
If you carefully note the clues, about half way through the film it strongly indicates the finale. After that the air of mystery that keeps the film feeling suspenseful largely dissolves and you are left with what feels like a AI-generated script desperately trying and failing to throw you off the initial clues.
The film's greatest redeeming quality is its attempt to turn some revenge film character archetypes on their head. It is not always successful, but the attempt to break cliches is always welcome! I suspect a much better film will be inspired by this.