Open challenge: pick any piece of art and explain what makes it good or bad without using any opinions.
jammindude is right with regard to the ability to say something to the effect of "
The Dark Knight is a good film because it has seven actings and ten writings and nine-and-one-half visual effects."
However, I think I can give reasons that a particular piece of art is good. So, if you insist, I will give it a shot. And if you don't mind, I'll use a film that I'm pretty comfortable with:
The film
Spider-Man 2 is a good piece of art because it effectively portrays the intended theme that "with great power comes great responsibility" through the story of Peter Parker, who is the masked, super-powered vigilante Spider-Man. The plot of the film shows Peter making decisions that reflect his commitment to the responsibility he feels toward the general public and the negative results that these choices have on his personal life (his work, his education his friendships, his romantic life). It then shows his change in attitude towards these feelings of responsibility as they continue to take their toll on his happiness, with the eventual decision to give up the Spider-Man identity. However, Peter then realizes the impacts of his decision on the people of New York, including his loved ones, and decides to return to his role as Spider-Man in order to protect the people of the city, even though he must give up his dreams in the process. This progression in Peter's attitude is believable since the series of events which befall him, none of which seem unlikely though a few make use of coincidence, would make a reasonable person with his attitude change viewpoints in the manner he does.
This theme is also expressed in the progression of the Dr. Otto Octavius character, who attempts to build a dangerous piece of technology—one that he has always dreamed of building—using stolen money. In the conclusion, Octavius realizes through the destruction that his creation causes and the words of Peter (who tells him that sometimes people must give up their dreams, paralleling advice that his aunt has given him and that he has taken to heart), that he must destroy his invention. Octavius's impulse to forsake responsibility is symbolized by an apparatus of mechanical arms that have artificial intelligence that are bonded to him. Octavius's shift in attitude is not presented as believably as Peter's, since the change is more sudden and fewer events precipitate it, but it is reinforced by the physical separation between his sense of responsibility and his desire to forsake it that the film portrays, in addition to the portrayal early in the film (before the arms are attached) of Octavius as a generally caring, responsible man.
The decisions of the character Harry Osborn, who despises and wishes to kill Spider-Man because he believes that Spider-Man murdered his father, further reinforce the theme. Late in the film, Harry is granted the power to kill Spider-Man when Octavius brings Spider-Man to him as a captive. Harry is ready to kill Spider-Man, even if it means killing his best friend Peter, until he learns that his cherished friend Mary-Jane Watson will die if he chooses to kill Spider-Man. Harry, too, opts for the rout of responsibility in realizing that he must use his power over Spider-Man to release him in order to prevent his friend from dying. Harry, also, gives up a dream for a worthy cause. This is a minor component of the overall action, but Harry's decision is shown as believable since it is clear that Mary-Jane represents a significant value to him (and since he is depicted as being not totally eager to commit murder).
The actors in the film generally do well at portraying the plot. Alfred Molina's performance as Otto Octavius is very believable and captures the essence of Octavius's struggle with his mad desire to complete his invention as well as his ultimate turn to a responsible course of action. Tobey Maguire's portrayal of Peter Parker is characterized at times by a more limited range of emotional expression, and is not as lively as Molina's, but it does portray Peter as a confused, uncertain young adult, which is a portrayal that complements the attitudes he is shown as possessing over the course of the film. James Franco plays the role of Harry Osborn well, vividly depicting the desire for revenge this character possesses, and his later confusion and hesitance towards the act of killing and decision of self-denial in choosing the path of responsibility. The weakest part of the acting is Kirsten Dunst, who plays Mary-Jane and, like Maguire, has a somewhat limited range of emotion. One of her and Maguire's weaknesses, that they sometimes speak too slowly, which can negate the dramatic tension of some scenes by pacing them too slowly, could potentially be chalked up to directorial decisions.
I am not qualified to comment on length on the directing and other visual elements of the film, but I will briefly say that throughout the film the visual elements do not detract and often enhance the immersion of the film. The Spider-Man web-swinging and wall-climbing scenes, in particular, depict with few visible lapses a realistic portrait of a man with such preternatural abilities.
Spider-Man 2 is a good film because it possess a compelling, believable plot (with only a few slight lapses) that expresses the intended theme of the choice that exists between personal desires and responsibility. This plot is depicted with mostly believable acting, though there are parts of the cast that are not as strong as they could be. The visual elements of the film do not at any time detract from the plot's believability and at times enhance it.
Edit: And I expect that you're going to reject this because it's "opinion-based." I'm not really sure what to do with such a rejection, except to say that there ARE objective truths to be stated that do not come down to a matter of numbers or of simple observations. "This plot is logical and believable" IS an objective statement that can be made, as is something else like "this actor accurately depicted this attitude or emotion." Just because I can't quantify the exact ways in which he did so does not mean that it is impossible to objectively state that he did it. So there's pretty much all I have to say about that.