I am finding it a bit hard to use the vertical scrollbar. Every time I need to scroll (which is often), I have to break my concentration and look directly at the scrollbar in order to locate it with the mouse pointer. 

I should only have to move the pointer in the general direction of the scrollbar or one of it's sub-objects, and let the pinning effect of the screen edges do the rest for me.

When multiple tabs are open, the inefficiency is caused by the 2px gap between the vertical scrollbar and the edge of the screen. So removing the gap would be nice, although there may very well be implementation difficulties with GtkNotebook (border widths and such).

For interested readers, Fitt's Law provides a deeper explanation:
http://en.wikipedia.org/wiki/Fitt's_law