What made the Cold War ''hot spots'' hot?

I am not sure exactly how you mean this
question.

Are you asking why they were called "hot?"  If
so, the reason is that these were places where tensions between the Soviet Bloc and the
US would flare up -- places where they could or did erupt into "hot wars" as opposed to
the Cold War.

Are you asking why some places became "hot?" 
If so, a major reason is because some of these places bordered on communist countries
and the communists were trying to expand.  This is the case in Korea and Vietnam. 
Another major reason is that some of the hot spots were in areas that were close to the
United States.  This is the case in, for example, Cuba and Nicaragua.  Others were in
places that were important for their resources (like the Congo/Zaire) or for their
location (like Egypt on the Suez Canal).