We have given even more love to line settings recently. Last week we made it look better when lines are crossing each other; this week it's about the start and end arrows.
There is now a stylesheet option to toggle if a line should have a start / end arrow.
You can also add arrows to the lines linking note boxes.
This can be managed from the stylesheet screen.
Brilliant, I can see this being used with note boxes to represent directional relationships between different concepts. A good example would be a causal loop diagram, or message sequencing.