I’m not a Joni fan at all - although I grant that the album “absolute court and spark” (???) was very influential and quite original (not negating my previous post).
Bit of trivia - Jimmy Page was/is a huge Joni fan - and apparently was quite influenced by her. Seems amazing to me but it’s the truth.
It was the guitar work.
Joni Mitchell was using some bizarre tunings, chords, and progressions for the time, and every guitar slinger of the time noticed it (including yours truly).
She also had incredible chops for a gal. She wasn't just strumming open chords and being a front for some folk band. She was a virtuoso.
I think that a lot of young girls were inspired to pick up the guitar by her. Today, you can find a lot of smoking female guitar players, but Joni was one of the first, first-rate female guitarists in modern popular music.