With the recent spate of money-making, blockbuster films that are heaping piles of suck, people complain that it's unfair to call a movie bad- it's just opinion. There are no objective standards.
There are objective standards. When picking a hammer, many of the details may be a matter of the user's taste, if the hammer is unable to drive nails, we would all agree that it's a bad hammer. If your aunt knits you a sweater with no arms, while it was a sweet sentiment, we would all agree that it's a bad sweater. If someone were to take a great number of words, selected at random, put them onto page and call it a novel, I think we would all agree that it's a bad novel.
While there is a lot of room for opinion, in any creative endeavor, there are metrics for success and failure. There are, mostly qualitative, but some quantitative measures for whether a creative work is "good" or "bad".
My purpose here is not to write a treatise on critique, which I'm sure has been done better by someone else, who has devoted a great deal more time and effort to the subject. But as a critical dilettante, here's how I approach a critical examination of creative works- regardless of medium.
The Message
A creative work exists to communicate something. The creator(s) have something they would like to impart to the audience. In a work of any complexity, they probably have many things they would like to impart to the audience. The first part of critiquing something is examining the message and deciding if this is a message that is interesting and meaningful.
There's certainly a great deal of room for subjectivity in this category. Most people would probably not enjoy a film that takes the stance, "Hitler was awesome". But, even if one disagrees with the message, one can still evaluate the message objectively. Is it an interesting message? Is it something original, or a novel insight into something familiar?
I'm not claiming that good works have to have some intense or deeply meaningful message. If the purpose of the work is merely to entertain, it still wants to communicate something more than mere entertainment to the audience- photographs have subjects, novels have plots, and so on.
So, the first to questions I ask myself are: what is the message, and is this message interesting or notable in some way?
Communication
Once we've identified the message, the next question we have to examine is: was the message communicated clearly? Did the audience understand it? Was it comprehensible to begin with? What would have made it more clear?
This is largely qualitative, but there are certainly some objective measures. Whether the target audience understood it can be measured pretty objectively by asking them. Things that don't support the message can be marked as superfluous.
Technical Merit
Each medium has its own methods, metaphors and techniques. Things that work in novels can't be done in film, painting has established practices. Given the nature of the medium, the final question is, Was this work technically competent? Someone can perform a piece on the violin, express it beautifully, but if they lack technique, this can be identified by an experience violinist (I, for example, lack technique). If a filmmaker or photographer ignores the basic principles of composition, this can be identified.
Of all of my categories of critique, this is the most objective. Every medium has principles and practices that are established through trial and error- people have found what works. It is possible that someone can throw those rules out and be successful at communicating, but it's pretty unlikely. (Andy Warhol threw out the rulebook, and while he was successful in the sense that he's mysteriously popular, he didn't really communicate anything but how little he had to say.)
And that's true for all of these categories. Maybe the story has no real point, but is entertainingly told. Or the film has no budget and looks awful, but carries you with its interesting ideas.
Some examples
This was prompted by an online debate over the merits of Michael Bay films. I love ridiculous action movies, but hate his movies. Since I haven't suffered through Transformers 2, lets look at the first one.
What was the message of Transformers? Well, its primary purpose was clearly to entertain, and to entertain as a vehicle for robot battles and explosions. Nothing wrong with that, but nothing particularly exciting either. The story moves forward on the back of a McGuffin, and events happen, none of them terribly significant.
Did it communicate this message well? Not terribly. First and foremost, the main reason for seeing the movie was robotic action sequences. There weren't many of those. What little action there was confused and annoyed, more than entertained. There were plot cul de sacs, like the NSA/hacker subplot which served no purpose at all, except to put another attractive actress in the film, I guess. There was a chain of unfunny physical comedy (I love physical comedy, but this was just bad) But, honestly, pretty much everything was a plot cul de sac. By the end of the film, I was aware that things had happened, and I didn't care one whit.
What about the technical merits? People hold Bay up as the master of the special-effects spectacle. The reality couldn't be farther from the truth. Every special effects moment was specially designed to be incomprehensible. The robots didn't look terribly much like robots, and they had no real structure or proportion to their design. The action sequences did their best to be disorienting, which isn't really a good thing to do to your audience. There was no sense of structure to any of the camera work, and it really felt like they weren't planned at all. The best analogy was that it's like fingerpainting with special effects.
Contrast this with a film like Crank, which I maintain is the gold-standard for action films. In terms of their core message, they're pretty much identical. Once again, everything about the film, from the thin plot on, is about getting into action sequences. Events happen, but there's not a great deal of significance in any of them.
But in execution, the differences really start to show up. First off, Crank keeps on message for the vast majority of the film. Each scene moves from action sequence to action sequence, with the odd ridiculous sex scene thrown in for fun. Plenty of physical comedy, often grotesque. While the main character is just another gruff talking growly scowly manly-man, he doesn't look the part, and that adds character. The supporting cast, the doctor and his girlfriend, carry all of the non-actiony bits, and everything is delivered with a sense of comic timing all the way around.
Technically, it does its job. Ridiculous fight scenes move quickly, and are pretty exciting all the way through. The camera is always in the right place, you always feel grounded in what's going on. It's not a virtuoso violinist playing through a Mendelssohn concerto, but a punk rocker laying down a bass-line. There's technique there, but it's not the focus.
I'm no art school film snob. I'm not a snob about most things. Sometimes, you want a nice filet mingion served with sauteed truffles and a nutty red with a strong finish. Other times, you want a hamburger. But just because you want a hamburger doesn't mean you're going to fill your gob with the predigested slop at McDonalds and say it's delicious.