To me, most important is the usage of "good old fashioned" elements of music: rhythm, melody, harmony or at least some sort of vertical organization. I generally like anything within contemporary idiom (atonality, aleatorics, clusters, sound layers) if music is active, energetic and has enough contrasts. My favourite composers of contemporary music are Olivier Messiaen, Henri Dutilleux, Marc-Andre Dalbavie, Erkki Sven Tüür, Magnus Lindberg, some works of Einojuhani Rautavaara.
I don't like composers such as John Cage, Morton Feldman, Earl Brown, Karlheinz Stockhausen because most of their compositions are static and boring.