What is the difference between set back limbs and mild R/D limbs?

Set back will bend at the handle, away from the shooter. kind of a straight line bend.
  R/D is a bending through the handle and up the limb(deflex) then switching to reflexing out to the tips. Deflex is toward the shooter and reflex is away from the shooter.

think of 'set back' as induced limb reflex only - the riser is straight.  a side view shows the limbs/tips slightly arcing in front of the riser section.

'r/d' (reflex/deflex) is limb reflex coupled with a riser that pushes out forward of the natural straight side centerline of the bow.  thus, a side view defines a 'snake'.  

mild r/d is just that - a shallow curved snake.  some mild r/d longbows exhibit a 'd' limb shape when braced (see mohawk longbows as an example).

aggressive (hybrid) r/d longbows have quite snakey limbs, increased performance, and won't exhibit a braced 'd' limb shape - there'll be a slight 'flip' or reflex seen at the limb tips.  EXCEPT for 'stealth r/d' longbows, such as the 21st century edge, where the aggressive r/d is tamed down to a 'd' braced limb shape.

Learn something every day...thanks Rob, always wondered about the differences myself.

Here's a little pic I drew up. I'm far from having any artistic or graphic skills but maybe this will show the difference.
 

Spike,
close, but change the "Deflex" label to "Reflex" and you've got it.

Most people find the mild (and aggressive) r/d bows easier to shoot than reflexed bows, especially the heavily reflexed bows (which are some high performing bows)  It has to do with where the hand position is in relation to where the limb and limb tips are.  The more forward you get your hand the less touchy the bow is to anything you do with it during/after the shot (torque, twisting, dropping etc).

That was a DUH! on my part. I changed it to Setback.

Thanks that makes sense.