I disliked the way sensors 'behaved' in Galactic Civilizations II; they had a fixed radius/diameter. In Imperium Galactica II the further you got away from your planets, the smaller the radius/diameter of the ships sensors became untill you had nothing left.
This slowed down the exploration of the galaxy significantly. In Galactic Civilizations III, such behaviour would also put a stop to the quick exploitation of various elements in the galaxy (minor races sending out constructors by the dozens comes to mind).
(Have I mentioned IGII too many times already? 
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