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.
And she played Martin guitars for the most part. I loved the full, rich sound (partly due to the open tunings she used). It was the early 70s, and I was in love with her guitar playing. I couldn't afford a D-28, so I bought a D-18.
Well if Jimmy Page was impressed - I don’t want to get into the “who’s greatest” argument here - but let’s just say that he knows his stuff :)