I think that one difference between what we call "classical music" and what we might call "popular music" or "commercial music" (including music written in the various idioms used for what we call "classical music") has a lot to do with the motivations of a composer. The same composer can, of course, write music that is intended to be commercial as well as write music that is done with little regard for how it sells (or is consumed). There have always been great composers who wrote both commercial (or popular) and non commercial music. I suppose it has a lot to do with who the consumer of the music happens to be, and how s/he consumes it.
I believe that commercial music is written primarily to engage audiences. Film music is written to manipulate an audience to react to visual images and narrative in a particular way. Opera is too, and even more so in the 20th century. What we call "pop music" is very intensely geared to appeal to an audience, and it is "consumed" mostly by people who are not practicing musicians. Vocal pop music is often defined by a singer (or THE singer) connected with a song rather than the composer of a song, unless it happens to be a rare instance when it is the same person. In the case of the singer-songwriter, the idea of writing the song in the first place seems to be about sharing an emotional experience directly with an audience, where it is often consumed like "pop music," but is then "covered" by people who want to sing it themselves (and often not for an audience).
I assume that I'm not alone when I mention that communicating something to an audience is the last thing I have in mind when I write a piece of music. I write music in order to communicate with musicians and allow communication between the musicians who are playing the music I write. It is their business to communicate their interpretation to an audience. My job ends (if all goes well) where their jobs begin.
I have read that Telemann's motivation for writing music was to give people music that they enjoyed playing. I believe that Bach's motivation was similar, but he also had a job where his music needed to demonstrate the teachings of the Lutheran church. He managed to combine the whole enjoyment of playing thing and the manipulation of spirit thing rather seamlessly. Even people who describe themselves as not the least bit religious are moved emotionally when they listen to Bach's religious music. Instrumentalists who play Bach's non-religious music find personal enjoyment in the process of practicing it. And the personal enjoyment is something that we experience daily. For most of us there is no need to perform Bach's music. We play it because of what it does for us in our own private spaces.
Haydn and his contemporaries published string quartets so that people could play them for collective enjoyment. The challenges and jokes that he put in his music are there to enhance the private musical discourse. When Mozart wrote his "Haydn" Quartets, he did so as a way of communicating with a composer he admired. I believe that the idea of performing them for an audience would have been the furthest thing from his mind. The commercial success for most of the composers of the Classical Period came from the sales of their music. It's kind of like the concept of commercial success for Milton and Bradley comes from the sales of their games.
Sure, Mozart and Haydn did write music that was intended for audiences, and those audiences were often important ones that included monarchs and patrons, but I believe that some of their "public" music was different from the music that they wrote for their friends and for musicians they admired. I think that they wrote chamber music and piano music primarily for people to enjoy playing, and I believe that they always wrote their orchestral music (particularly Haydn, who had standing relationships with the members of his orchestra) so that the musicians would enjoy themselves while playing it. In Beethoven's later years he cared a great deal about what audiences thought of his Quartets (like the late Quartets), but those people were often musicians themselves, and Beethoven was an exception to all rules.
Moving forward to the period of uncommon practice (a.k.a. today), composers write their music for musicians to play and enjoy. The idea of the standard musical ensemble has expanded greatly. There are a great number of percussionists who like to use large numbers of instruments. There are people who enjoy using sophisticated electronic instruments, people who have expertise using extended techniques, microtones, and multiphonics, and people who enjoy exploring non western scales. Many of these musicians are young, and many are interested in playing new music written by people of their generation. The existing repertoire would be very small (if existent at all) for an ensemble made of tuba, percussion, flute, harp, bassoon, and French horn, and for such an ensemble to survive as a performing group, it would need good new music to play. I like to think that composers who would write for a young ensemble like the hypothetical one above would be more concerned about how playable the harp part is, if the bassoon can be heard above the tuba, or whether the percussionist can participate in the ensemble without dominating, than how an audience will respond to his or her musical ideas.
Members of the listening public and consumers of recorded music should remember that there is a lot of new music that is not being written for your enjoyment. It is being written for the enjoyment of the intended performing musicians, and for the enjoyment of other ensembles made of the same instruments as the intended ensemble. If they succeed at communicating the essence of a piece of new music to you, they have succeeded in their mission. If they don't, it could have something to do with the composer (who, if given a chance to hear a performance, can correct the problems in the work), or it could have something to do with errors of judgement (or notes, or rhythms) made by the performing musicians.
I really don't believe that contemporary composers should be judged against composers from a previous eras. I don't believe that living composers should be judged against dead ones. (The living composer can grow, and the dead one can't.) I also don't believe it is fair to judge a composer from the "common practice" period against a composer from what I call the "uncommon practice" of the present day. There will never be another Rameau, J.S. Bach, Haydn, Mozart, Beethoven, Schubert, Brahms, Schumann, Debussy, Bartok, Ravel, Stravinsky, or Shostakovich (to give just a few examples). These were all singular musical personalities who continue, long after their lives ended, to challenge our imaginations and remain relevant and modern. I believe they all speak to the musicians who play their music now with the same voices they spoke with while they were living, but as "consumers" of music (everyone reading this post is probably, on some level, a consumer of music) we need to keep an open mind about what the music of today means to today's young musicians. The best stuff will survive because people will want to play it, and the stuff that nobody wants to play might have to wait for another generation to find musicians who appreciate. It could also simply fade into oblivion. Only time will tell. While we wait, musicians will still enjoy playing music written by the composers on the above list and by composers who may have not been as well respected or well known during their lifetimes.
I'll end this little rant by paraphrasing a statement I heard Stephane Wrembel make about the influence Django Reinhardt has on him: you can work under the shadows of great composers, or you can be illuminated by their light.