Did a lot of playing around with both of these versions today and I'm liking 9.1 better. If you're debating on going to 10, make sure these won't bother you.
Action Window
In 9.1, you can detach it and make the tree list more viewable. In v10, every new thing you open, it makes another window. So in order to save space, you have to close the other ones which defeats the purpose as that takes more clicks. Also editing in 'full screen' is easier on the eyes. I suppose you could make the treelist bigger, but again, defeats the purpose as you use more clicks, more looking, and more headaches.
If v10 could use 1 window and still be able to switch easily between open actions, it would be better. I still would like to see it to detach and remove itself from the tree.
Columns
Seeing '...' all over the place is kinda messy and gets old after awhile. I like to see the whole field when scrolling through the list. Manually sizing columns doesn't work because you have to make them wide and then you waste space which means more scrolling than there needs to be.
The bold font is a nice touch because you're able to see the current track better.
Panes
New 'Field', Checkboxes....not my thing when just playing stuff. I'd prefer this be shown only when in 'Tagging Mode'. It clutters up the view. IOW, more info is being shown than there needs to be.
That's about it. I'm sure there might be other areas too. These were the big ones for me. Hopefully 10.1 will have some options to turn these things off.