The problem with music recommendations
There's a popular song, written by an Argentinean musician called Charly García, called "Los Dinosaurios" ("The Dinosaurs"). The song was released in 1983 in the album "Clics Modernos", and you can listen to it in all its vinyl glory here.
This song represents for me an interesting problem: it is by far my favourite song from this author, and I would like to listen to something similar. But so far all recommendation systems have failed me. Here are some of the reasons why.
A first approach could be to pick something from the same author, or even the same album. This approach, sadly, doesn't work: while Charly García is certainly a prolific author, with 41 published records and countless guest appearances, his main style is oriented towards electronic music, and it doesn't really fit the style of this specific song. If anything, this song is more fitting for his earlier albums, which limits us quite a bit - out of those 41 soloist albums, "Clics Modernos" is the second one.
We could instead assume that this song was written in a certain context, and that looking at authors from a similar context we can obtain similar music. Again, this doesn't entirely work: if we pick "Argentinean songs from the 80's", we would end up with a list of songs that fit perfectly the style of the other 8 songs on this album, but not this one specifically^1. Grouping the song into "Latin American music", as some systems do, only exacerbates the problem: there is no relation at all between this song and, say, a Cuban bachata.
If we look at the actual lyrics, the situation gets even worse: "Los dinosaurios" is a thinly veiled critique of the military dictatorship that ruled the country between 1976 and 1983. A lyrics-based systems would most likely fail on two fronts: either it wouldn't understand the references made in the song and label it as "nonsense/fantastical", or it would understand the reference and recommend politically charged songs. Neither approach seems really right - while "The times they are a-Changin" could be a viable candidate for a recommendation, neither "The Revolution will not be televised" nor "Redemption song" fit the bill.
All of these approaches fail for the same reason: they apply a network-oriented measure to a song that doesn't fit the popular rhythm of the time and place in which it was produced, and which doesn't fit the overall style of the author either.
So what exactly am I looking for? A non-technical answer would be "I need a song that contains simple vocals, a piano as it's main instrument, and with raising tension towards the end". Or in the words of the author, a song that "adapts the English sound to Tango". As far as I know, the only system that applied a similarity measure capable of detecting this would be Pandora, but with their system closed to Europe, I cannot tell whether this works or not.
Footnotes
^1 How to obtain a digital archive of Argentinean songs from the early 80's is left as an exercise for the reader.
