The US finds out about attacks ahead of time the same way we always have: through intelligence operations, not data mining. I don't think anyone is saying that targeted, legal wire tapping is somehow off-limits now. I don't see how this would impact the ability of a company to assist the authorities in monitoring individuals' communications, either - it will, possibly, make it more difficult for the NSA to do a blanket gathering of foreign communications, which I believe is the point.

Large service providers like Google and Amazon can and will comply with the laws. It is possible that social media start-ups will be unable to operate across borders due to regulations, but this will hardly be a staggering setback to the European populace's ability to share photos of their food. I doubt there are any real implications for things like freedom of speech/expression: these types of services are already effectively illegal in places with heavily authoritarian governments.