What were the negative effects of WWI?

In my opinion, by far the most negative effect of WWI was
WWII.  The first war led pretty directly to the second.

The
First World War led to a situation in which Germany (especially) felt the need for
revenge and the need to upset the status quo.  The Treaty of Versailles that ended WWI
took much of Germany's territory away from it and imposed crushing financial reparations
on it.  These led to a desire for revenge.  Germany also wanted to put an end to aspects
of the treaty that prevented it from having a serious
military.

By punishing Germany harshly, the Treaty of
Versailles led Germany to have a motive for starting WWII.  This makes WWII the main
negative effect of WWI.