The one thing I didn't like...

Was when Merrick jumped back in time. Why would he appear in his own earlier body? Likewise the other two future people. If you can only appear in your own body, then the future tourists shouldn't have been able to come back since they weren't born yet.

Other than that, I thought the movie was very well done. Quite a bit of moral ambiguity. No real bad guys, just various people trying to do what they think is right and best for the world.