There's so much that I haven't tapped. This is a really awesome board.
Things I love:
The new voices - the vocal S.A. and Megavoices are awesome
The ability to have three voices. Not only can you get new textures, you can get more volume in those horns.
Lots of great styles + all of the Tyros 5 and Genos styles that now sound incredible.
The software is a great improvement
It's really great that you can freeze the modulation wheel setting and save it as a registration + you can make it so that the modulation only applies to your organ and not to additional voices.
The vocal harmony software is easier to use
The sub outs are pretty nice. If I'm recording keyboard audio, I don't have to worry about screwing things up my adjusting the keyboard volume to my tastes.
The keyboard is easier to pick up.
The touchscreen has a lot of advantages.
The second USB is very convenient. I'm keeping all my T5 & Genos files permanently plugged into the keyboard with a mini USB + backup registrations in case I lose my USB at a performance.
I like the assignable buttons and live control knobs. I like that when you turn the knob, it says what level you're at. I'm keeping these knobs and buttons fixed to one setting, and I'm putting labels underneath so I know what they're for.
Oh, the half fill capacity and the reset capacity are going to be invaluable for those songs that have a 2/4 or 3/4 measure inserted - like Thank God I'm a Country Boy, Strawberry Fields Forever, etc. It's about time!!
The things I miss from the S970:
Overwriting a registration was much easier. Now you have to go through a lot of steps if you just want to make a minor change to a registration and re-save it in the same location.
I miss having the mixer right there on the home screen.
I miss those gray buttons for increasing style, mic, and voice 1 & 2 volume. I miss being able to reset levels to 100 by pressing both up and down at the same time.
The S970 was actually a little easier at making changes to the mix on the fly.
Even though the assignable buttons are great, the SX900 took away some buttons that were on the S970 like Fade In/Out, Effect On/Off, and Text viewer.
I miss a dedicated menu of just R&B voices, just Country voices, just Swing voices, just Ballads. Yamaha screwed this up IMO.