There are even more hotkeys that aren't mapped by default. You can see them all in your hotkeys.txt file, found by clicking the button on the settings dialog.
Pan Move and Zoom Move are hotkeys where if you are holding the hotkey down and move the mouse, you get pan or zoom. This can be useful when using a pen. Pan Drag and Zoom Drag (J and U by default) are similar but require the left mouse button to be held.
Last Tool swaps to your last used tool (right click also does this). This makes it more convenient to switch between two tools.
Hide Selection can be very useful but isn't mapped by default because hitting it on accident would be extremely confusing. When something is selected, it hides the selection outline, mesh lines, the tool icon, bones
everything. This lets you see the skeleton clearly while making adjustments. Deselect so you can see things again to make a new selection.
There is also a Double click option on the setting dialog where you can turn off double clicking. Some users were annoyed by accidental double clicks when using a pen.
