I would argue that it's not about "songwriting" per se as much as a total commitment to expression that exceeds almost anybody else while still being pop. That, combined with her fantastic voice -- which is way too high for most pop and only works because she has a tremendous instrument and really wraps it around the lyrics. The fact that she sings over her whole range is both the hardest thing to get used to and one of the things that makes her a giant.
Finally, it's what she represents, which is an artist who was able, over and over again, to pursue her exact vision while giving zero fs what EMI wanted -- and was able to release top selling albums (in the UK) in the 70s, 80s, 90s, 00s and 10s.
She's probably the only pop artist that is worth engaging with as a genuinely mind-broadening experience that should be undertaken precisely becauseyou might not like it at first, like some classical or jazz.