The amendments to the Rome Statute differentiate between "crime of aggression" (committed by an individual) and an "act of aggression" (committed by a state). An act of aggression must occur for the ICC's jurisdiction to be triggered, but only the crime of aggression falls under the ICC's jurisdiction (naturally, since it is a criminal court dealing with individual criminal responsibility). The definition of the act of aggression is taken directly from the United Nations General Assembly resolution 3314 (XXIX) of 14 December 1974 adopted by consensus by the collective body of the world's sovereign states.
Check out articles 8bis, 15bis and 15ter of the Rome Statute: http://www.icc-cpi.int/nr/rdonlyres/add16852-aee9-4757-abe7-9cdc7cf02886/283503/romestatuteng1.pdf
The ICC defines the crime of aggression as "the planning, preparation, initiation or execution by a person in a leadership position of an act of aggression." Just because a person, leader or nation plans or prepares a crime does not mean they will ever commit that crime, and planning and preparation are private behavior that should not be legislated. The crime of aggression should be limited to initiation or execution of an act of aggression.
The International Criminal Court does not have a clearly defined definition of aggression. Because the definition is so subjective, it can be interpreted in any way that the users want to interpret it. Therefore, it depends more on whether the members of the ICC disapprove of a country's actions than whether the country has clear cross adequately defined boundaries.
They may think that since they accept hearsay as evidence, that should be considered as being aggressive. May be it should be more aggressive towards the more richer and powerful countries. The fact that they do not allow jury trials might be considered as being too aggressive to some people.