Question

How did WWI affect Europe and the U.S. differently?

Answer 1

I would say because a difference in terms of loss of lives and destruction.

Explanation:

WWI was basically fought outside the U.S., in France (and Belgium) and Russia (the western part) with destruction of large parts of the countries involved.

Even today in Flanders or the region of Verdun you can find "cratered" pieces of countryside while many French farmers still dig out shells or other grim relics in their fields.
This destruction involved cities, farms, railways, road networks, factories and caused the dismemberment of countries (Austria-Hungary, Poland/Russia, the Balkans, the Ottoman Empire) and forced dislocation of populations (with terrible consequences for the future).

The loss of lives among the belligerents was also different:

These losses were going to impact on the economies and social structure of most of the nations of Europe creating recession, turmoil and political instability (as for example, in Russia, Germany and Italy).