Despite the incredible variety of styles within the world of classical music, great composers share certain consistent qualities that make their music great.
Great composers write from the heart
Effective composers don't try to razzle-dazzle you with fake flourishes. They mean what they compose. Look at Tchaikovsky: This guy spent half his life in emotional torment, and — wow! — does his music sound like it.
Mozart was an incredibly facile composer — melodies just bubbled out of his head effortlessly, and his pieces reflect that ease. Stravinsky was a strictly disciplined, calculating, complex character; ditto for much of his music. Although their personalities were incredibly diverse, these composers wrote great music in a way that was true to themselves.
Great composers use a structure that you can feel
Great pieces of music have a structure, a musical architecture. You may not be consciously aware of the structure while you're listening to a great work; but still, you instinctively feel how that work was put together. Maybe the piece follows one of the classic overarching musical patterns (called things like sonata form or rondo form). Maybe it just has a musical idea at the beginning that comes back at the end. In any case, you'd be hard-pressed to name a great work of music that doesn't have a coherent structure.
Recent studies at the University of California showed that students who listen to Mozart before an exam actually score higher than students who don't. (Of course, these students probably would've scored higher yet if they'd actually studied before the exam.) As you listen to a piece by Mozart, your brain apparently creates a logical set of compartments that process this form. These compartments are then useful for processing other kinds of information as well. Classical music actually does make you smarter.
Great composers are creative and original
You hear again and again that some of the greatest composers — even those whose works sound tame and easily accessible to us — were misunderstood in their own day. Not everyone could relate to the compositions of Beethoven, Brahms, Mahler, Strauss, Debussy, Stravinsky, or Ives when those works were first composed. (Actually, that's the understatement of the year; the audience at Stravinsky's The Rite of Spring actually rioted, trashing the theater and bolting for the exits.)
The reason for this original lack of acceptance is unfamiliarity. The musical forms, or ideas expressed within them, were completely new. And yet, this is exactly one of the things that make them so great. Effective composers have their own ideas.
Did you see the movie Amadeus? The composer Salieri is the "host" of this movie; he's depicted as one of the more famous non-great composers — he lived at the time of Mozart and was completely overshadowed by him. Now, Salieri was not a bad composer; in fact, he was a very good one. But he wasn't one of the world's great composers because his work wasn't original. What he wrote sounded just like what everyone else was composing at the time.
Great composers express a relevant human emotion
Great composers have something important to say. They have an emotion that's so urgent, it cries out to be expressed. The greatest pieces of music (any music, from rock to rap, from Sinatra to Selena) take advantage of the ability of this art to express the inexpressible.
When Beethoven discovered that he was going deaf, he was seized by an incredible, overwhelming, agonizing frustration. His music is about this feeling. He expresses his frustration so clearly — so articulately, in a musical sense — in every note of his compositions. Beethoven's music is intense.
Now, this isn't to say that great composers must be intense. Joseph Haydn, for example, exuded cheerful playfulness in almost everything he wrote. Like all effective composers, he had something significant to say, too.
Great composers keep your attention with variety and pacing
Effective composers know how to keep you listening. Their music is interesting throughout.
One technique that achieves this effect is variety. If the composer includes in a piece a variety of musical ideas, or dynamics (loudness and softness), or melodies, or harmonies, he's much more likely to keep your interest. In this way, a great piece of music is like a great movie. An explosion near the beginning gets your attention, right? But have you ever seen a movie with an explosion every minute for two hours? (Okay, okay, besides Independence Day.) Have you noticed how each explosion becomes successively less interesting, until finally you don't even notice them anymore? You need variety — something contrasting and different between explosions.
In a movie, one explosion can be thrilling if it's approached correctly, with a suspenseful buildup. Effective composers know how to use dramatic pacing, too. Their music seems to build up suspense as it approaches the climax. Maurice Ravel's Boléro (made famous by the movie 10) is a stunning example. The entire piece of music is one long crescendo (getting louder and louder) — the suspense builds and builds for 15 minutes, and the climax is shattering.
Great composers' music is easy to remember
In today's pop music world, the word hook refers to the catchy, repeated element in a piece of music. Beatles songs are so catchy because nearly every one of them has a hook. Think "Help!" or "A Hard Day's Night" or "She Loves You" ("Yeah, Yeah, Yeah!"). Catchiness is not a scientifically measurable quality; still, you know a hook when you hear it.
In classical music, the same concept applies. A hook helps you remember, and identify with, a particular piece of music. The compositions of Mozart, Tchaikovsky, Chopin, Rachmaninoff, Bizet, Dvorak, Gershwin, Grieg, and Schubert have hooks galore — so many hooks, in fact, that several of them have been pilfered for the melodies of today's rock songs. Barry Manilow's song "Could It Be Magic?," for example, is a Chopin piano prelude with words added — Barry didn't write the original music. And "Midnight Blue" is sung to the tune of Beethoven's Pathétique sonata. The music of the most effective composers is full of elements that stick in your mind.
Great composers move you with their creations
The most important habit of highly effective composers is their ability to change your life. Ever walk out of a movie or play and suddenly experience the world outside the theater differently? You know, when the real world just after the movie seems to have a feeling of danger, or sadness, or happiness, or just plain wonder, which it didn't have before?
A great musical masterpiece may give you a greater appreciation for the potential of humankind, or enhance your spirituality, or just put you in a great mood. Nothing is more triumphant than the end of Gustav Mahler's Second Symphony; after you hear it, you emerge reborn, refreshed, somehow more prepared to face the world.