I've been finding that I want to scroll both vertically and horizontally quite frequently. I find it non-intuitive that I can scroll horizontally on the gantt chart using the mouse but have to move over to the tree and use the scroll wheel to scroll vertically. This leads me to twiddling the wheel while I'm in the chart and zooming by mistake.
It would be a big improvement - for me anyway - if I can grab and scroll the chart both vertically and horizontally. Either this, or have the scroll wheel consistent in function in the chart and the tree.
I also find myself on occasion absent-mindedly moving the horizontal scrollbar on the tree to move the chart.
In other words, there seem to be a bunch of inconsistencies in navigation between the tree and the chart and I often get muddled by them.