Let's look at Feldman's approach to musical elements.
In articulation, he emphasizes decay over attack:
This is perahps why in my music I am so involved with the decay of each sound, and try to make its attack sourceless. The attack of a sound is not its character. Actually, what we hear is the attack and not the sound. Decay, however, this departing landscape, this expresses where the sound exists in our hearing--leaving us rather than coming toward us.
In dynamics, soft rather than loud. And very, very soft, ppp.
In tempo, slow rather than fast. He liked the tempo of about 60bpm.
In structure, development, his music is static rather than directional. There is little or no narrative or movement forward.
In rhythm, duration rather than ictus.
In the construction of phrases, isolated notes or clusters rather than linear phrases.
In musical gesture, nuance rather than bombast.
In timbre, a kind of ghostly presence rather than the stencil or stereotype of the sound.
In other words, he has a consistent approach to music evident in all of these dimensions, so that if you put loud, fast, articulated, developmental, etc... in one column you would find he avoids all of those things. His music is the anti-bebop.
What I haven't gotten to yet is his approach to melody and harmony. The exam was only an hour and 15 minutes. I would say he that his sequences of short clusters are anti-melodic. He doesn't compose very many tunes that go up and down the scale. Harmonically, the music does not build tonally in the classic style, nor does it follow a 12-tone, serialist technique. What I don't know yet is what it does do.
These are very crude judgments so far. I'm just trying to figure out what I know so far, and it could be that what I know is partially mistaken.