But the weird effects of QM are observed in systems that are (temporarily) isolated from the environment.

... Is the double-slit experiment isolated from the environment? http://en.wikipedia.org/wiki/Double-slit_experiment

--yes, more precisely, quantum mechanics is free to be weird until it decoheres, and it depends on the environment and the phenomenon, how long that takes. However in most human situations the world seems non-quantum and non-weird, which is since the decoherence times are smaller than human perception time scales in nearly all situations humans get to see with naked eyes.