from Slate Magazine:

Miles Davis' Kind of Blue, which was released 50 years ago today, is a nearly unique thing in music or any other creative realm: a huge hit—the best-selling jazz album of all time—and the spearhead of an artistic revolution. Everyone, even people who say they don't like jazz, likes Kind of Blue. It's cool, romantic, melancholic, and gorgeously melodic. But why do critics regard it as one of the best jazz albums ever made?

and on Wikipedia:

One significant aspect of Kind of Blue is that the entire record, not just one track, was revolutionary... "It wasn’t just one tune that was a breakthrough, it was the whole record. When new jazz styles come along, the first few attempts to do it are usually kind of shaky. Early Charlie Parker records were like this. But with Kind of Blue [the sextet] all sound like they’re fully into it." Along with The Dave Brubeck Quartet's Time Out (1959) and Coltrane's Giant Steps (1959), Kind of Blue has often been recommended by music writers as an introductory jazz album, for similar reasons: the music on both records is very melodic, and the relaxed quality of the songs makes the improvisation easy for listeners to follow, without sacrificing artistry or experimentation.