Feldman's music is quiet and slow and atonal. He became inextricably associated with one musical direction often written in his scores: "as softly as possible." People will either love his music or hate it, I would guess. I'm in the former camp.
And I am in both camps -- I love Rothko Chapel and say, his sparse works for two pianos but don't care much for Coptic Light and am struggling right now with Patterns in a Chromatic Field (despite the new release on Tzadik with Aleck Karis).
Tim Rutherford-Johnson's take on Rothko Chapel.