First off was the semi-abandonment of a conventional key structure. Chords were strung together almost randomly. The predominant chord pattern was I VI VII iv or I VI IV or inverting it by going III I (which is the doorbell sound but rarely used in music) etc which was pretty much foreign to other bands before Nirvana... but exceedingly common in songs by Pearl Jam (chorus from I got ID), Soundgarden (The Day I Tried to Live and Blow Up the Outside World), Stone Temple Pilots (bridge in Big Bang Baby), Screaming Trees (All I know), and Hole (Celebrity Skin among many others). The sound was given with a loud and sloppy delivery completely different than the polished hair metal bands of the era with lots of distortion and creative feedback.
Next was the subject matter. Various weird and taboo subjects that took square aim at boomers, rednecks, popular kids, and the establishment.