I just noticed a couple of more thing that are in Komplete 9 that are not in 8.
Traktor's 12 - some GR5 effects aimed at EDM producers and DJ. Some of the effects could have uses in other musical styles but its not something that would tip the balance for me.
The Giant -That has got me a little more interested for sure. A piano sufficiently different to be interesting, but sufficiently conventional to be used a lot.
I got "The Mouth" with Komplete 8 but I can's see it in Komplete 9, but I think it was thrown in as a special for a limited time.