I've written 29 posts about new features in 8.5. And now core team decided to name it 9.0. Great. And now I have to make changes in all of these posts.
Just kidding – change is simple – I just added tag “pg90″ and that's all.
Change from 8.5 to 9.0 emphasizes significance of new features in this version, and generally looks cool marketing-wise.