A huge part of the Nirvana sound is dynamics; clean, quiet verses followed by loud, distorted riffs, which was inspired by bands such as The Pixies. Kurt leaned towards writing very simple songs and enjoyed both pop music and heavy hardcore underground bands. Classic acts such as the Beatles and Neil Young were huge songwriting influences.
Lead wise - many of Kurt's guitar solos simply play the melody of the song. This is a hugely underrated technique that often leads to very memorable guitar parts - definitely worth practicing!
Key playing points;
Dynamics - super clean riffs that suddenly burst into loud distortion
Simple chord choices but that often elicit an instant emotional response from the listener
Solos often play the melody of the verse or chorus
Affordable Guitar Pedals for sounding like Nirvana