We don't like steep RWY gradients and that would make opposite departures very bad; anyway most RWY's are inclined some what
further, would you like to stop a fast jet running down hill,...
and if the runway were unidirectional well then you save lots of fuel,...by idling on the ground forever waiting for a departure slot,...or holding forever waiting for an approach,...
just another useless 'new idea'---let's just float away on pink balloons and hope the wind carries us home
