It wouldn't be any of the things I have mentioned previously: FOR loops, FLOOR and CEILING functions, different method for handling layers.
It wouldn't be the facility to clone sprites. It wouldn't be the facility to write subroutines and functions.
Note: Some of these have been addressed in Scratch 2.0 which is currently in Beta and all of them would be on my wish-list.
Instead it would be to reverse a decision which I'm sure the designers made with the best of intentions. It would be to allow you to move a sprite completely off the "stage".
The designers decided obviously that having a sprite which if off the stage would be confusing for kids. So if you instruct a sprite to move off the stage it will only do so partially - the x/y coordinate will simply refuse to increase/decrease past a certain point.
This is annoying where you want to:
1) Have a sprite glide onto or off the stage
2) Work with scrolling backgrounds
So yes: if I could do one thing then that would be it.
And no: this has not been changed in version 2.0.
This picture shows how you cannot get the Scratch Cat to exit the stage no matter how many times you press the space key:
