Suspension parts may be a bit much, but I've had good success with smaller parts for my race bike. Like brackets for a CO2 regulator that would have been difficult in metal due to angles necessitated by the curve of the fender. http://www.daves-speedshop.com/CO2brackets.php Making my own velocity stacks for the throttle bodies allowed me to add mounts for a sensor and the airbox tray. http://www.daves-speedshop.com/graphics/FT550/sparkPro2.jpg I couldn't make those out of metal without a full CNC mill.

Would 3d printing work there? Isn't there a whole bunch of crystallization hoo-ha around metals and how they are made to make sure they can handle high stresses? e.g. knives being folded many times; rotor blades in jet engines grown from a single piece.

Exactly. 3D printing makes an approximation of the 3D space occupied by a given part, and nothing more. For some parts this is sufficient, for most it is not.

There is a whole history of materials science and production techniques to get to the 'stuff' we have today, 3D printing is rather 1 dimensional in comparison.

This comment goes a bit too far the other way. 3D printing is a manufacturing technique with its own possibilities and drawbacks which you need to be aware of, but it's very possible to make engineering-grade parts with 3d printing - for some things, it's even the state of the art. You can eliminate layer stress lines with annealing, and you can print interior shapes that would be impossible to injection mould. I myself have made polycarbonate fan blades, only a few mm at the thickest point, with a tensile strength tested to over 500 kilograms. Their theoretical strength is much higher still.