We got 37 million deaths, millions and millions of bright people, a death toll so high that it skewed the demographics of nations such as France.
Really just a tragic, colossal waste. However, it is quite possible that it would have been even more horrible had it been fought 10-20 years later (with better technology).
I read one author who said the exact time of the decline and death of western, Christian civilization was when the two-sides soldiers returned to their trenches after spending Christmas together in no-man's land. Before that was hope and confidence, after that moment, only pessimism and death.