Updated: Jun 17, 2020
The short answer: Its completely subjective and everyone has their own opinion.
A better question would be what are the schools of thought on the quality of a motion picture. Its easy to say ,"Everyone has their own opinion" but the problem is some movies are (almost) universally considered good and some are universally considered bad.
We could talk about specific traits of a given movie, is it funny? Is it smart? Is it thrilling? Is it sad? Not all films are trying to be one of those things and these wouldn't be fair gauges to measure the quality of a film.
This all seems pretty trivial to someone who creates or critiques media for a living, but among those who examine film in this way, there are a number of different factors considered or not considered to determine how good is a movie.
Can a movie be good and be boring. In my opinion, yes. I know that sounds like the opinion of an art film snob movie purist that sticks his nose in the air and brags about all his favorite movies of which you've never heard. But, let me ask you, which is the better movie: Titanic or Avengers: Endgame. Now, same movies, which one is more entertaining? Your answer probably isn't the same. Watching people hurt themselves on Youtube is undeniably more entertaining than say.. A well thought out Ted talk from a Harvard professor, yet the richness and quality of one is clearly higher than the other.
There's an answer to that and it goes back hundreds if not thousands of years. Moby Dick, The Adventures of Huckleberry Finn, Lord of the Flies. They're long (That's not necessarily what makes them good) they can be considered hard to read but they're also considered some of the greatest novels ever written. They're much longer than an episode of Full House or a children's book and what enables a long story (Despite maybe being boring) to be beloved and celebrated is the payoff. You've invested time and attention to get to know and (hopefully) care about the characters, so it hurts more when they die or it means more when they have a happy ending. You have more time to set up better jokes with greater payoffs because it was set up an hour ago and a lot has happened to take your mind away from it, so its more unexpected, making it more funny. The problem with summer blockbuster action flicks, typically a clumsy tapestry of boobs, explosions and motorcycles graced with "Directed by Michael Bay" across the lower third is that they lack depth. Its fine that these movies exist, they make money, give people jobs and they're certainly not meant to plant the idea in someone's head that they should crawl through an air duct using only a zippo for light.
They are not what makes a good movie.
Does a movie need to have a deeper meaning, does it need to teach a lesson and/or ask a philosophical question in a sophisticated way to be considered a good movie? This is a difficult question for me to answer because I make Music Videos around a hidden meaning. Its hard to find a feature film or novel of fiction that objectively doesn't have a subtext. Even Non-Fiction documentaries are beginning to introduce symbolism and narrative based on the content. So does it have to have a specific theme like "don't be a dick", "family is more important than work", "learn from history"? No, it doesn't have to. Does it have to ask a question like "Who do you want to be?", "Are you really a good person"? I don't believe it has to, no. Does it have to be an expression of emotion? I think it has to be at least one of those things. Otherwise its completely empty and in my opinion defeats the purpose of making a feature film. If what your looking for is cheap, meaningless entertainment there are lots of ways to accomplish that without a 2 hour runtime and $100 Million budget. However, when a film focuses too much on subtext, it risks becoming "preachy" , "uninspired" or "shallow" especially when they're misguided. Christian films immediately come to mind, with a very narrow point being made that clearly panders to a small demographic. While filmmaking shouldn't focus on marketability, a good movie will have a more vague subtext that can be interpreted in more ways than one and more people can agree with, and marketability is actually the only thing Pure Flix movies have in mind, cornering their demographic and usually confirming their prejudices which they care so deeply about is a great way to sell tickets, but not a great way to provoke thought and emotion.
Its too easy to assume that the image/sound quality of a film doesn't matter at all, but I think that's just because its always been a given that a feature film is going to look better than an SNL sketch. This is very rarely the issue with bad movies (Atleast in recent years), in fact its really common to see reviews that says something in the way of ,"While visually stunning, lacks in yadahyadahyadah" but you never see a review that says "No plot, terrible acting, worst direction ever but there is kick-ass slow motion so 9/10". Visuals can add or take away, but can't make a movie. While filming Jaws, Steven Spielberg and crew captured unprecedented, incredible underwater footage of numerous sharks in the same spot at the same time. This footage was not used in the movie, because it would have changed the story OR.. made the existing story make no sense. Either of these decisions may have ruined a classic movie and nobody would remember a shot that can now easily be created with CGI and in fact, has several times in movies of which you probably have never heard. While cinematography does not make a movie great, a film that does not present audio and visual creatively, might as well be a book, and as long as it doesn't compromise your plot/performances/dialogue/etc. creating artistic looks and sounds can only improve a work.
So now I have to answer the question: why are some films universally Loved while some are universally hated. If it were 100% subjective that couldn't be the case with any film. Its about the ability to balance these three elements and the many specific characteristics that make them up. And I do mean balance, you can't just keep adding more and more ingredients to the pie. If you focus too much on entertainment value your film can become cheap, too much on subtext and you might as well just come right out and speak your point, too much on cinematography and you might be compromising important plot details for a shot that looks "Bad-ass". Having said that, there is no perfect formula, if there were there would be no bad movies. Even movies that seemingly everybody adores, will have a few critics that think they're a little too.... something. Some people just want hours of exciting nonsense, that's why Godzilla exists. Others want films that make them feel sophisticated if they get to proclaim that they understood Interstellar. Movies, like all things subjective are made differently for different people, but what I love about the medium, is the fact that the ones that everyone generally likes are not typically the equivalent of the "pop" genre in music. People don't always flock to see the cheap Steven Seagal movie where he fights a shark or some shit. Well, sometimes, but those aren't the movies that people remember.
"The Dark Knight", "The Shawshank Redemption", "My Girl" "Saving Private Ryan" "Ferris Bueller's Day Off"
These are films that everyone says ,"Now that's a good movie" across many different genres and decades after their release. Not because they were all dark or funny or filled with action but because they balanced being entertaining, meaningful and artistic with competence and diligence.
And that's all I have to say about that.