"most of Mozart's music is dull"- Famous opera singer Maria Callas.
"Mozart was a bad composer."-Reknowned pianist Glenn Gould
I personally wouldn't go as far as Gould, but I can certainly sympathise with the ever estimable Maria, and not just because she had a fascinating voice. Mozart's music and I have had our fling, one that was ended upon my discovery of Joseph Haydn, late-era Beethoven, and of course Richard Wagner. I'll expand on the reasons shortly, but first allow me to give you some backround.
Several years ago I came under the spell of the movie Amadeus, and went on a bit of a Mozart kick. I bought a bunch of expensive recordings, checked out most of his operas, hung up a picture, etc. Six months later, after listening in depth to Joseph Haydn, as well as late era Beethoven and Mahler, I realized how repetitive and unoriginal that Mozart could be.
Try giving a concentrated listen to his works, early to late. Though obviously masterpieces, he almost never changed the underlying structure. To put it bluntly, Mozart beat to death the same sonata form that Bach, Handel, Haydn, and the Italian masters had already mostly exhausted. The same turnarounds, repetitions of whole sections, theme, development, recap, etcetera...all the stuff that Beethoven blew past with his Eroica and opus 59.
The string quartets that Mozart dedicated to Haydn are another great set of examples: despite the stupendous quality of craftsmanship, and godlike sense for melody, how much different are those quartets structurally from Haydn's opus 20 (or, ironically, Beethoven's later opus 18)?
It's true, there are many staggering examples of Mozart's genius: the Divertimento in Eb, Don Giovanni, and the Requiem are just a few that come immediately to mind. But, for a specific example, listen to the last symphonies he wrote, specifically nos. 35-41. In fact, try setting aside a few hours to hear them back to back. The melodies are amazing, some of the most dazzlingly brilliant in history. His sense of dynamics can be incredibly inspiring, in fact he was a master of light and shade...but, all within the boundaries of the form he adhered to. He rarely deviated. In fact, his resourcefulness in the face of using practically the exact same structure ad infinitum is astounding. Of course, a mitigating factor that must be kept in mind is that Mozart was a hack, like most composers of every age are (nothing wrong with that, ya gotta eat!). There were political variables that demanded the same-y style and sound of so many of his compositions.
Joseph Haydn, Mozart's contemporary, is given consistently lower ratings than Mozart (not too low, of course, Haydn is widely acknowledged as a spectacular composer, and rightly so). But Haydn practically invented, and most certainly pioneered, the string quartet and symphony. No kidding. Mozart carried the ball from the real trailblazer of his generation: Joseph Haydn. This is not meant to take away from the awe-inspiring genius of Mozart, who (with Joseph Haydn) perfected the whole sonata form of his time. Even Beethoven was so under the spell of Mozart that almost all of his early work, and quite a portion the middle, was indebted to him.
Please understand that I tend to look for different things in a composer than do most other people. Joseph Haydn, late era Beethoven, the operas of Wagner, to a lesser degree the mid to late era works of Gustav Mahler...those men just pushed the boundaries. They were aiming toward things that hadn't existed, that were considered unacceptable. And they achieved things in their own language. Granted, most of those composers were given more room to create in their respective political climates.
Still, it's hard for me to listen to anything by Mozart after listening to, say, Beethoven's opus 127. Actually, that particular piece is so fantastic that it's hard to listen to anything else after it, so maybe I'm being unfair (laughing).
This is all of course just my opinion, the opinion of an accomplished modern day composer without even an iota of the popularity of Mozart. I completely respect those who disagree, and I'll be the first to praise all the fabulous music Mozrt has written, the list is obviously much longer than I've given space to here.