Why I Don't Like Mozart
Most people don't really hold opinions on classical music one way or another, and those that do generally see it as monotonous and boring. I've never really held this opinion, but my favorite music has always come later in music history - not with the dissonance and atonality so revered by the composers of the 20th century, but with the Romantic era.
I'm using "classical" in a broad sense because I'm really not the world's biggest fan of music from the Classical period. Those who aren't as familiar with this history may be a bit lost at this point, so I'll see if I can't briefly recap some of the details.
The Middle Ages and the Renaissance are more or less grouped together in one era of music history. This was a very long era, but there were crucial innovations in harmony, melody, and musical notation. By the high Renaissance, polyphony - multiple melodies at once - was extremely common, and the best composers were able to write motets that used up to 40 distinct voices. Polyphony was music.
Around 1600, as musical instruments increased in quality and secular music became a more popular genre, the Baroque era started. Baroque music is often characterized as architecture, and Baroque composers were, as a general rule, ridiculously prolific. (Telemann is still the single most prolific composer in history, and Vivaldi nearly got thrown in an asylum when he interrupted himself at his day job - as a priest - to write down some notes that had occurred to him.)
Baroque music still drew on the polyphony of the high Renaissance to a certain extent, but by and large this kind of writing wasn't very common. Most Baroque composers used one or two melodies, with the notable exception of Johann Sebastian Bach. His keyboard music - especially his complex fugues with their finger-breaking polyphony - was considered antiquated, and his sons (he had a whopping 20 kids overall) were considered better composers than he was when he died. His reputation was revived when his works were rediscovered in the mid-1800s, and now, he's the only Baroque composer most people are familiar with.
All of which brings us to Classicism, where musical form became a bigger deal. Instrumental sonatas, symphonies, concertos, and string quartets became standard forms, and methods of writing for those ensembles were also standardized to a certain extent. Essentially, if you have a theme or two, you could plug those into sonata form, add a few basic harmonies, and boom, you've got a sonata movement. Simplicity and clarity became the name of the game in the Classical era.
The three major composers of this period were Joseph Haydn, Wolfgang Amadeus Mozart, and Ludwig van Beethoven. While many like to group their music together as the "First Viennese School," the music of these three were different. Haydn, speed-writing symphonies for the court orchestra under the Esterházy family, was, as he said "forced to be original," although many of his 100+ symphonies are not particularly innovative, as he had to conform to the musical tastes of both the Esterházys and their guests. His contributions to form have long since outlived him, and due to his productivity and his standardization of forms, he is known as the "Father of the Symphony" and the "Father of the String Quartet." Haydn's contributions to form and the language of Classicism cannot be understated.
Beethoven was widely different - while he started out fixed to Classical molds, he experimented with pushing the limits of what said forms could handle even before he realized that he was going deaf. When he came to terms with this, his experimentation led to more innovative and trail-blazing music, dispensing with convention after convention. He replaced the minuet with the scherzo in his later symphonies, looked towards Romanticism with his Sixth Symphony and a great number of his piano sonatas, didn't stick to traditional movement numbers in his late piano sonatas and string quartets, and famously introduced a chorus in the last movement of his Ninth Symphony.
Now for Mozart.
Mozart was hailed as a child prodigy, composed prolifically, and died at the young age of 35. In that time, he stuck to the already well-defined Classical forms, choosing to do as much within those constraints as he could. However, there really wasn't much more that any composer, no matter how great, could do within those forms - forms that were already well established by Haydn by the time Mozart began composing.
Haydn, though his music contains an aesthetic similarity, was creative as a musical troll. His Symphony No. 94 - nicknamed the "Surprise" - was designed to wake up sleepy members of the court with a massive chord following a very soft passage. His Symphony No. 45 - nicknamed the "Farewell" - sent a message to his patrons to let the musicians return home by letting players leave as the last movement progresses. Haydn chose the unusual F# minor as the symphony's home key, and had to get special crooks for his orchestra's horns to play.
But Haydn's formal unoriginality is explicable, as we know that he had to compose musically conservative pieces in order to get paid and did quite a bit under those kinds of restrictions. Mozart was, for most of his time as a composer, not hindered by a particular court. He was, more or less, freelance. Financially insecure, Mozart had the opportunity to be an innovator such as Beethoven came to be later, but did not.
Mozart, for his part, did write a few brilliant pieces in his later years - his unfinished Requiem, his Clarinet Concerto, and his late symphonies among them - but in his entire oeuvre these masterpieces are relatively few. If you took all of Mozart's works, put them into a list, and then randomized it, chances are you're not going to come out with one of his great works. Most of his pieces have a similar mood, and as mentioned, they nearly always stick to a predetermined form. In this sense, a great many are interchangeable.
In the end, this all comes down to my personal preferences and musical tastes. I know that Mozart holds a special place in many people's hearts, but as someone who has listened to a wide swath of his pieces, I really don't see what all of the fuss was - and still is - about.