Simple theory of IMDB bias

IMDB top 250 list is dominated by old movies,

With 72 out of 250 movies form the last 12 years, it appears that there's more of a bias towards the present. Perhaps more movies are being made these days - or perhaps people prefer current moveis.

new movies are far better than old movies Premise: I don't necessarily agree with this point.

Rate Your Music shows a similar phenomenon, though. The enjoyment of music is in some respect different from the enjoyment of movies, and - if the reason behind the behavior of the two charts is the same (the hypothesis sounds reasonably plausible) - we can maybe identify what the two have in common. Looking at your list of possible causes, they don't look perfectly like they could give a possible explanation of origin of RYM's chart, too, but there are indeed some interesting considerations.

For new movies, very wide population have seen it, many not fans of the genre. They vote on IMDB soon after watching

Music fans are different beasts: they usually mix novelties with classics in their discovery of music. It's still true, though, that you have to more actively search for older music, and therefore are inclined to listen to music that generally meets your tastes. This can be similarly true for movies: you usually go to actively search for an old movie if you like the director, some of the actors or the genre, and it's probable that you will like the movie in the end. On the contrary, massive advertisement of new movies may convince you to see one that in the end was misrepresented by the trailers and doesn't meet your tastes.

For old movies, only narrow population of fans have seen it recently. The only people who vote on IMDB are those who've seen it recently (atypical fans), or have particularly good memories of it (atypical fans again). People who watched an old movie ages ago but don't remember much about it are very unlikely to vote on IMDB.

This isn't usually true for albums. Music fans periodically listen to their favorite albums, even if they are really old. They should have therefore always a "fresh" (more on this later) impression of the recording. Something resembling these effects happens if you look for niche genres (e.g. extreme metal): in this case the only people who listen to those albums are the fans of the genre, and the score for the albums are usually artificially high.

Other effects that I can see playing a role:

• cached beliefs: people tend to form an emotional bond to movies and records they loved in their teens. They tend to keep this kind of judgement even after years have passed, and tend to value newer movies/records less. "All this new music sucks!"
• social cached beliefs: if something is deemed to be a masterpiece, it's more difficult to dismiss it with a bad rating. "Ok, I don't really get Citizen Kane/The Velvet Underground and Nico, but I respect them /recognize their historical importance".
• various level of signaling/countersignaling: very common indeed. If you are seen as a fine critic and expert, how can you admit that you prefer Scary Movie 3 to Apocalypse Now? Or the Spice Girls to Coltrane? "How can you listen to that crappy pop music? Come with me and enjoy some good latin-avant-garde-techno-art-brutal-classic Jazz".

Disagree with the premise. New movies tend to have more plot holes, less characterization and worse writing. Improved effects only rarely make up that margin. I also find the following stories just as plausible as yours: "New movies are over-represented on the IMDB top 250 because they get bolstered by excited fans who just saw the film and haven't yet taken the time to digest the movie or see how it dates and who, often, haven't seen the old movies on the list." The Return of the King is not better than Blade Runner.

/done with my silly arguing for the day.

Did a quick google search on number of movies made each year; actually it looks like modern movies versus old movies have about the representation you'd expect if quality of movies was essentially constant (the number of extra modern movies on the list corresponds approximately to the number of extra movies made.)

majority of people as far as I can tell

Low likelihood of sampling bias here, obviously.

Why is correct in scare quotes? If it isn't the correct solution then what is it? On a website called Less Wrong correct doesn't exist? Such is the mental schizophrenia that relativism brings.

Correct is in scare quotes but if what's being recommended isn't correct then what is it? On a website called Less Wrong there's no such thing as correct? Such is the mental schizophrenia that relativism brings.

For practical reasons this cannot really be done[.]

Really? It seems practical to me, at least for enough movies to demonstrate bias or lack thereof in IMDB ratings.