I thought Berlin has many airports (I remember Tegel and the other one). So this is another new one ? If yes, why is there a need for a new one :) For example Frankfurt has only one and it's capable much more than Berlin, because it's also a Hub for Lufthansa.
Tegel - "New" West Berlin one - not big enough
Tempelhof - "Old" West Berlin (Airlift airport) - closed
Schonefeld - Soviet built crap. BER is being built next to it