The album overall is just different from a lot of whats going on in (mainstream) hip hop right now, and it's also a concept album, so you hear it a few times and start putting the story together and understanding/possibly relating to more of it the more you hear it.
I know a lot of people on this board are Kendrick haters, even though I'm sure most of them thought Section 80 was god's gift to hip hop when it came out, but you can't deny that Kendrick has sent a lot of rappers home rethinking their whole shyt, and setting the bar higher for hip hop as a whole.
Long story short, GKMC is a great album in an era of hip hop thats looking the best it's ever been since the early/mid 90's