It's sublety - the ability for whatever hardware you go for to reveal some facet or detail of the music you've never heard before when playing it on lesser equipment ... on my "reference" setup (old Technics class A amp, MOR Marantz CD & 25+ year old monster Leak speakers) I can hear whole extra _instruments_ compared to my
PC setup (NF& onboard sound & Creative 5.1 squeakers).
Genres? Lots really, techno, eurphoric, lots of subspecies of house, rock, opera, classical, skiffle, reggae, ska, dub, goth, acoustic, choral, pop, hip hop, chillout, funk, some country ... & probably lots more that didn't spring immediately to mind.