Angles start with zero being rightwards, in the direction of the positive X axis, and increase clockwise. 90 degrees, for example, goes downwards, in the direction of the positive Y axis.
It is the SelectionElement that is rotated, not necessarily the whole node. Often one wants to rotate the principal element(s) of the node, but not the text or other decorations.
You can enable user-controlled rotation of nodes by setting Rotatable to true. That makes use of the Northwoods.GoXam.Tool.RotatingTool, which may set this dependency property. Therefore one-way data-bindings of this attached dependency property to updated source properties may not get updated values when this property has been set.