Nirvana's sound is no longer that unique by contemporary standards, but the reason for that is that many of today's popular rock bands completely styled their sound after bands like Nirvana and Pearl Jam and that sound has since become so widely adopted and modified that it detracts from the originality of the innovator when heard by modern listeners. I'm not the biggest fan of Nirvana's work save for a few standout songs, but I can at least recognize why they were so influential at the time and remain so popular with fans, a sentiment similar to my opinion of the Beatles except I can't fucking stand the Beatles.