During the course of my career as a music lover, musician, and self-anointed musical pundit, I have gone through several different theories about the difference between "good" and "great" fine-art music. Here is my current thinking in a nutshell:
A "good" composer writes music of consummate skill and craftsmanship: intelligently structured, attractive, well-proportioned and -balanced, that professional musicians can play without shame. A "great" composer writes memorable music in such an individual way that no one who has heard a reasonable amount of fine-art music is likely to mistake one of his pieces for the work of another composer.
There are, to be sure, composers who are both "good" and "great." Alas, there are some who are only one or the other. Some may argue, for example, that Bruckner and Sibelius were not consistently "good" composers. The one wrote in a fussy, insecure, and sometimes tedious manner; the other often produced works that seem a bit sketchy and difficult to understand. But no one denies that they were "great" composers; and I think a strong proof of this is the fact that the instant you begin to hear a piece of music by either of them, you immediately know who wrote it. You can shake your head at Bruckner's rustic plodding or Sibelius's half-baked eccentricity, but even if you can't quite like their music, you somehow love it and respect it.
On the other side, take Mendelssohn and Bruch. I used to be very impressed with their mastery of musical forms and textures. There is a kind of exquisite perfection in pieces like the Overture to A Midsummer Night's Dream, Mendelssohn's Italian and Scottish symphonies, and the violin concertos of both composers. Some of their music may, indeed, qualify for greatness. But Mendelssohn's piano concertos seem to glide right through one's consciousness without leaving a ripple; and in his early string symphonies, he does such a spot-on imitation of late Haydn or early Schubert that only the lack of wind instruments clues you in as to who done it. I wonder if such a perfect mimic can also be a truly original artist. Bruch's symphonies are also quite nice; but they are so heavy with the odor of Schumann and, yes, even Mendelssohn, that it's hard to think of them as original works.
Of course, composers are only human. They all had their off days. Often a composer will create many good, and great, works of art, while also producing items that are neither good nor great. I doubt Beethoven's "battle symphony" will ever have many fans. If Schubert actually wrote the E-major symphony I posted about some time ago, I pity him. And really, just because Carmen is the greatest opera ever doesn't mean we should have to listen to Bizet's Symphony in C, which is really not so hot.
Fine-art music, like rock'n'roll, has its share of one-hit wonders, such as Carl Orff (Carmina Burana), Paul Dukas (The Sorcerer's Apprentice), and Pietro Mascagni (Cavalleria Rusticana). I also has a number of composers whose every album seemed to go platinum (such as Wagner, Verdi, and Brahms). It has its geniuses who are so far "ahead of their time" that they are only really appreciated by later generations (such as Bach and Mozart), and its crowd-pleasers who are mostly forgotten after their moment has passed by (such as Telemann and Salieri). And some of fine-art music's "greatest hits" pass all understanding. Take Franck's Symphony in D minor: a study in ponderous monotony, broken up by flashes of garishly bad taste, it yet remains in the "standard repertoire." I could describe Gershwin's Piano Concerto in F minor in almost identical terms, yet it always seems to be near the top of KFUO-FM's annual listener-generated "Top 99."
There are composers whose music used to thrill me, but as my tastes mature I find them less and less appealing. Chief among them is Tchaikowsky, whose music once called out to a passionate spirit within me, but now seems rather crudely put-together and obvious in its manipulativeness. Then there are composers who appeal to me more and more with every passing year, such as Haydn and Bach. There are old friends who are always comforting to go back to, such as Schumann and Schubert; and there are new heroes for me to tag along behind with wide-eyed adoration, such as (most recently) Walton, whose first symphony I have heard about six times in the last month, and for some reason just now I "totally get it."