Art

Art, categorically, is mimetic expression; that is, all art is an imitative representation of an idea: an object, or sensation, or sound, or event, etc. Accepting that criticism differs from stating an opinion in its attempt to ‘improve’ a work of art, it follows that true criticism, then, deals largely with an artist’s perceived ability to represent an idea.
Every artist, however, and also every critic, has their own unique perspective.  This is the trick of criticizing art; for if everything is relative, then so is improvement, and so for whose benefit is this improvement, the artist, the critic, or a universal audience?  One could say that a "good" critic makes suggestions that one can agree would ‘improve’ a work of art.  

This cohesive desire, the satisfaction of agreeing with a critic on an improvement, implies a desire for finding a universal quality in art that any who are willing should be able to appreciate.  The first and foremost goal of the critic is appreciation; if a critic does not attempt to understand the representation, no conclusions about the effectiveness of the work can be made.  Relative perspective cannot be avoided, as it shapes all synthesis, but a critic must be wary of what appeals specifically and what appeals to the audience.
It is impossible to assess effectiveness without considering purpose (how can something be effective if it serves no function?).  The debate on the purpose of art is timeless and scopeless, but rather than estimating its intended function, let us instead observe and describe the effect of "successful art" upon the audience and assume that what occurs from "successful art" through the reaction of the audience IS the purpose (which seems a fair assumption, even if an over simplification, since both artist and audience are satisfied by "successful art"). 

This effect can be referred to as katharsis, from the Greek, translating into either clarification, purification, or purgation; there are enthusiasts and dissenters for each translation, but my interpretations are that all three are integral to the purpose of art: the successful artist imitates an idea and produces a representation of it, and in doing so, purifies or refines aspects of the idea seen as needing emphasis, purges the ‘foreigness’ of the idea by confronting it, and through this purification and purgation the artist and audience both get a clarification of the idea.  In this manner it can be said that purgation deals with the scope of an idea (the size and perspective of it governs the aspects of the work the artist and audience are confronted by, i.e., a work of art cannot deal with what is not presented by it, either actually or in implications such as a title or caption) and that purification deals with the focus of an idea (in refining the facets that are represented, the artist no longer affects what is being presented so much as how what is being presented is synthesized or interpreted).

The role of the "good" critic, then, is to promote the universal accessibility of a work of art without obscuring the original representation.  In producing the work the artist confronts the idea to purge the ignorance of it, and in being presented to the work the critic does the same.  The effective critic should concentrate on the purification of the idea, then, because the ‘angle of approach,’ or desired boundaries of an artwork, as it were, are part of the purgation and is the artist’s domain - the critic must correctly interperet the idea an artist seeks to purge the ‘foreigness’ of and instead of altering the perceived original idea should endeavor to focus more clearly upon it.
  

The difficulty in criticism lies in the discretion between the focus and scope of a work of art: it is the artist’s prerogative to provide the scope of a work with enough focus to represent the idea in a way that can be recognized accurately by the critic - it is the critic’s duty to accurately recognize the scope represented and to analyze and improve the focus so as to make each audience member’s relative synthesis as close to the artist’s original represented idea as possible.