The story is concise and precise, while extremely original and relevant to today's biological research. It has no plot holes as far as I can tell, answers all questions in a satisfying matter, poses interesting questions in the first place, and you just feel like the filmmakers knew that they were doing.
On the other hand, the acting is a bit bland (I believe that is the point, however, as it is a dystopian future). And... that's it, really. One of the best Sci-Fi movies I've seen yet.
In a sentence:
An airtight package of philosophical questions, character development, surprises and futurism. Can't really think of any negatives other than "it won't sweep you off your feet".