You talk about the acting being believable, for instance - but believable to whom? Because that's going to vary from person to person. You singled out Alfred Molina's portrayal of Doctor Octavious as being particularly believable, but I actually thought Molina's acting was generally one of the weak points of Spider-Man 2. He had one shining moment (I love his delivery of 'brilliant but lazy' near the end of the film, with that little smirk on his face), but his performance was otherwise largely unimpressive to me. At time I honestly found it laughably bad.
So who's right? If there's some objective truth to the equation, then one of us must be objectively wrong.
Or we just experienced Molina's performance differently because we're different people, and both of our experiences are equally valid.
I would imagine that one or both of us is wrong. I would guess that we're looking at different aspects of his performance in evaluation it. Perhaps one or both of us is remembering some aspects of his performance incorrectly (i.e. maybe I didn't commit some of the weaker parts to memory or blotted them out because there were parts that I really liked).
Just because these things are objective does not mean that they can be known by the means of a simple computation or even a simple judgement in a set of criteria. Judgement of the arts, since it is more difficult to gauge than in other areas, is also far more prone to human error (and face it: we didn't exactly make our judgements of Molina's performance in
Spider-Man 2 in the way one would want to in order to eliminate error—I'm willing to bet neither of us watched the movie again in preparation for our posts).
I think it is also important to reiterate the fact that those of who believe that the arts can be objectively judged draw a distinction between objective quality and personal preference. We've given examples like these many times before, but I'll give another one quickly here. I like Star Wars. I like
Attack of the Clones. If someone said to me, "hey, wanna watch
Attack of the Clones tonight?" I would be all over that. There are also people who hate
Attack of the Clones. My personal liking of it stems from the fact that it is a Star Wars movie and that I personally enjoy the way the latter two prequels tell the story of Anakin Skywalker. However: I recognize that
Attack of the Clones is objectively a mediocre film. The pacing is not the best and there are some seriously poorly written and poorly acted scenes. I do, however, think that because of the quality of the Anakin Skywalker progression,
Attack of the Clones is not as bad a movie as a lot of people think.
Now, here are a few key details: my evaluation of
Attack of the Clones, where I try to determine the quality of the film, is separate from my personal liking of it. Maybe I would rate it as a 5/10 film, but my enjoyment of it is 7/10. Another important detail is that my rating of it is not equivalent to the quality of the film. I called it mediocre. I said that it is a piece of art that does some things well and some things poorly. Someone else might evaluate it on the whole and find that it does more things poorly than I thought it does and says that it is worse than mediocre, that it is a bad film. Just because either of us says this does not mean that we are right.
Attack of the Clones is a piece of art that exists in reality, which means that it does have an objective quality level. But because there are so many facets of this film, it's not really possible to determine exactly how good or bad it is. This is why you can only make objective statements about something like this as broad generalizations. I can say pretty safely that
Attack of the Clones is worse than
The Dark Knight and better than a movie I made in my basement starring my dog (he's a bad actor, and I'm hopeless as a director), but it's hard to say whether
Attack of the Clones or
The Phantom Menace is the better film. Even if there is one that I can say I like more.
What a movie review is, basically, is an attempt to give a rough estimate at the objective value of a movie compared to other movies. A critic has to do his best to be objective and not let his personal likes and dislikes blind him to the actual strengths and weaknesses of the film. My "5/10" rating for
Attack of the Clones was my attempt to do so. It's hard to say how well I did, and this is where opinions and discussions and debates come in. For example, some might say that I let my personal liking of the film skew my review too favorably and that 3/10 might be a better rating for it.
That actually brings me to a relevant point: the statement that art has objective quality doesn't mean that we can just measure it and say "okay, done." It's hard for anyone to study every aspect of a piece of art, put aside all of their personal likes or dislikes, and come to a solid judgement on its aesthetic merits. Because of this, debates over the quality of a particular artwork are common, welcome and good. You're looking at different aspects of the artwork and trying to decide whose measurement of its quality is most accurate.
Aside: as hard as it is to judge the quality of a film, it's impossibly harder to judge music. With film, we have a vocabulary for these things. We know generally what makes a good or bad plot, good or bad acting, good or bad directing, etc. With music, we don't have a conceptual language of this sort (yes, we do know about melody and such, but I don't believe we know nearly as much about what makes a good melody as we do about what makes a good plot). I would argue that because of this problem, we can't say anything much more sophisticated about the quality of a musical composition than saying that Beethoven's 5th is good and a band consisting of five off-key tuba players (or:
St. Anger) is bad.