Originally Posted by cyclezen

given that you don;t really have any significant structural differences between your 2 sides, then:
having the left cleat back, as noted by others, means more engagement of the quad & hip, less of calf and glut. more forward of the right side, easier on quad, more on glut & calf.
together this could also mean 'compensation' for an existing muscular imbalance, a left side which is not as strong/balanced as the right. meaning having to engage the left quad more to get a more balanced pedal stroke from both...

other consideration is, cleat back reduces the comfortable cadence - cleat forward aids more spin. Having them oppose each other would seem counter-productive. But then I don;t know for sure. Never heard of anyone I've spent time riding with who did this...
DO you know what your comfort cadance range is? what is it?

If you want to see the effect on your pedal stoke - try riding with only one leg pedaling, at your comfort cadence, for a mile or 2... then switch...
how'd that go?

whatever gets you down the road... of course the imbalance wouldn't be or allow for much longterm improvements in any area of the physical act of cycling.