Obviously a lot depends on taste. And taste is mostly a function of education. You tend to like the music you grew up with. But then, I do think that there are some objective parameters to judge the real quality of music, and I have clashed dramatically against people who think that there is no way to put objectivity and reason into art.
Despite the risk of being considered a technocrat, I will write down
some of these ideas now. I'm not a music scholar, and I'm not the owner
of all truth of this world, so don't take the following as an absolute
thing. Take it just as a simple, very incomplete set of ideas and basic
facts, put together by yours truly, in a vain attempt to explain why I
like some music and not another one.
We probably agree on what a sound is, and probably too on what a rhythm
may be. But what about a melody? What characteristics does it need to be
more than just a sequence of sounds? I think that one of the main characteristics
needed is proportion, and perhaps even logic! But then we come to the most
difficult concept: Harmony. It refers both to the harmony between individual
sounds, and to the harmonious, aesthetic, construction of the whole thing!
The next step would be to play two notes which are in a 2:3 ratio of frequencies. This is the case, for example, when playing C2 and G2 at the same time. The third harmonic of the C2, and the second harmonic of the G2, will both be the same: G3. At least, this is true so long as you use a perfect tuning. With the usual modern equally tempered tuning, it is not exactly true, but very close. If you play the notes quickly, you don't notice the beat note that forms...
How far can you go? The ratio between C and F is 3:4, and sounds
well. Between C and E you have 4:5, and it still sounds OK, although a
bit stretched. But play C and D together.... oooops! Ugly! They have a
ratio of 7:8. The shared harmonic is so far up that the beat note between
the two frequencies predominates. You no longer hear them as two notes
with an harmonic relationship, but rather as a single note with a very
strong, ugly modulation on it.
If you can do it, try to play C and D together on an instrument that is very well tuned to the perfect, nontempered scale. You may be surprised! The real 7:8 ratio does not sound that bad actually! But when playing the two notes on a typical modern instrument tuned in the equally tempered scale, the 7:8 ratio is too imprecise, and there are many other combinations of harmonics that are also almost coincident, but not exactly. This results in the disturbing, ugly sound of C + D.
Music sounds pleasant when all notes played at a given time follow this basic rule of having common harmonics that are not too far away. This is called vertical harmony, as it applies to notes written one on top of the other on a score. Additionally, there is the horizontal harmony, referring to the need of caring that the sequence of notes doesn't go against the harmony rules. This is most important if the notes will be played on an instrument with long decay times, or in a room with long reverberance, but is still important enough even without these conditions, since the human brain has a long memory...
Now I do not mean to say that it is forbidden to play a D right after a C! But doing it will not help the harmonic structuring of the piece! So you will need to care for enough harmony, in both dimensions, when composing music, but you are free to carefully fit in inharmonious sequences, as long as you do it discreetly.
Now, can you use vertical inharmonic stacking too? Well, many modern
composers do it way too often, and even old masters used it regularly,
but there is a basic aesthetic rule: When you raise your listener's hairs
with such an inharmonic stacking, you need to acknowledge what you did,
and follow it by a very harmonious one, which solves the created tension
and lets your listeners know that you can do it right! For an extreme
example of this, listen to the last notes of "Odi et amo", in any well-sung
performance of Carl Orff's "Catulli carmina"!
So, the whole idea behind tonality is selecting a set of harmonic relationships
between notes, and stick to them. In practice, when playing on western
instruments in the usual system, that has 12 equally spaced notes in each
octave, it means using only 7 of them. You pick them according to certain
rules, and off you go composing. In other systems, and other parts of the
world, and in other times, often more, or less, than 7 notes per octave
were picked, but the basic need of looking for harmonic relationships,
and picking a selection of them, has been the same, in all times and at
Now the question arises of how rhythm relates to the quality of music. And here comes a bold statement: In good music, rhythm never overpowers melody. While the rhythm is an absolutely necessary part of any music, it has no right to destroy the melodic flow, but rather must help it, underscore it, support it! Melody is the master, rhythm plays the servant's role! And another bold phrase: In good music the rhythm is implicit, without any need for a drum to mark it.
A corollary of the above is that at least 95% of all music composed in the second half of the 20th century is bad music. Sorry if someone out there feels attacked, but this is my view of the situation. I just need to hear a piece of music where a drum set is endlessly hammering the same rhythmic sequence, drowning out any real music that could have been below, to discard that music for being bad quality.
Rhythm must be varied. A musical piece that clings to the same, uniform, unchanged rhythm for several minutes, and then ends without further elaboration, is bad music, period. It may serve a purpose, as marching music or to demonstrate drabness or desperation in a movie, but it can never qualify as good music!
Any music that needs a drum to make the rhythm audible is poor music. A drum can be used, of course, to mark an emphasis, to support a musical peak, or to create effects, but never, NEVER, to uniformly hammer away the rhythm! That function is reserved to the infamous metronome, used by music students for practice, but a metronome has nothing to do in a performance!
I have heard some Gypsy music that applies these principles in a marvelous
way, using highly sophisticated, ever changing rhythms, that support the
melodic flow, and never need to be hammered out, since the rhythm is always
apparent from the playing of the melodic instruments. To a lesser extent
(less variations) this applies to most "classical" music, say, what comes
after chant and before rock...
How does it affect musical quality? Well, I like a thundering organ
sound, I love that hollow purity of the baroque flute, I can't resist
certain human voices, and I enjoy the ever new creations of synthesizer
artists. I can't think about any sound that is improper for creating good
music. The secret lies in the principles of harmony and proportion that
must govern the combination of sounds.
The greek philosophers tried to define it. Later philosophers tried too. Earlier ones too tried. And it's still hard to define. I only can get together a few ideas about this hardest task of all, and write down how I see them applied to music.
One of the best resources is repetition. The human brain works by making connections. Something known usually appeals more than something unknown. This can be used in making anything more acceptable, more agreeable, by first presenting it, and then insisting in showing it again! After a few times, the brain learns, and we find the repeated object to be attractive, or at least more acceptable than we found it the first time.
This is easy to try: Take an instrument, or sing if you prefer. Do just a sequence of any 6 notes. Whatever you like, as long as it is not a familiar melody. How does it sound? Well, probably like just any sequence of any 6 notes. No special beauty in it.
Now repeat it, but backwards. And do the whole 12 notes, again. Wow, a simple melody! It sounds more known already!
Now take your original sequence, and vary on it. Mirror it, or put it up by two or three notes. Even the transposed sequence now seems nice! Go on, vary it, don't loose the original contents, and finally return to the starting note. You have just composed music, following some basic laws of beauty: Symmetry and repetition!!
Almost every composer works in this way. You probably know that Ludwig van Beethoven wrote an entire symphony just on four notes, three of which are the same: The famous G-G-G-Eb. Just listen to his 5th symphony! Or let's go the the greatest master of all in this art: Johann Sebastian Bach. To show that he could write music on whatever simple theme he had to, he based the entire "Art of the Fugue" on such a simple theme, modestly choosing the sequence B-A-C-H!
So, a healthy amount of repetition, alteration, and re-use of items is a necessary ingredient to achieve a harmonic structure in music. But the repetition is often overdone! In a certain piece of modern music I have heard exactly the same phrase of 4 notes, repeated 64 times, without any variation at all! That's no longer music, not even bad music! That's just noise!!!
A musical piece has to start from somewhere, develop into something, and then end in some way that leaves you with the impression that the piece has actually finished, rather than leaving you thinking that for some reason the performers suddenly died. A much resorted trick to round off a piece (not only in music, also in literature!) is taking up an idea from the very beginning and re-use it for the end. In music, this often takes the form of ending the piece with a note or chord that's the same, or closely related, to the one used at the start. While being cheap tricks, these kind of things help a lot in bringing a harmonious structure into music!
But a lot more goes into making good music. There is a world of concepts about proportion, development, presentation and elaboration of ideas, and so on. Many books have been written about these things, and I will not try to rewrite them. And then there this other factor, that cannot really be learned, and makes the difference between a technically competent composer who writes unattractive music, and a real genius who creates masterworks: The gift of having the right ideas at the right time, coupled with the technical ability to set them in music, and the willingness to actually do so.