World building: implications of counter gravity

I am doing some world building for a space game setting, and want to run
 
an idea past people.

My setting is not too distant future. I want easy surface to orbit
launch	
to explain why people are in space, which means using up a lot of energy
 
on each launch.

BUT once in space I want engines to be rather limited, so it isn't easy 

to, say, divert asteroids into planets.

My idea is counter gravity, an updated version of HG Wells Cavorite, or 

the liftwood in Space 1889. Not artificial gravity, but some kind of
field  
that INSERT HANDWAVING HERE creates an equal and opposite thrust
reaction.  
So within the gravity of a planet you get lots of thrust, near an
asteroid  
very little, and from a spaceship hull something only measurable in  
nanometres per hour.

What am I missing? Would this make space travel economical? What else  
would it be good for?

-- 
	 cheers,
	 Hugh Fisher