I've about 60 miles now on the SPD-SL's, moving from loose toe clips and tennis shoes, and always was toe-down push forward pedaling. Now with the clipless, Flyingblind9 noticed I'm dropping my heels, possibly even past level on the backside of the stroke, and my leg gets straightened out.
I'm pretty comfortable right now with seat height and fore-aft position, able to spin even 110 + without bouncing if I concentrate. The shoe cleats are initially in all-back position, so the pedal shaft is behind the ball of my foot, and have not tried moving the cleat forward yet.
Is the problem just technique, or are there fine adjustments to the cleats or seat positioning that contribute to technique? The part that surprised me is toe-clips was toe down forward push, and clipless is heel down in back and (wonderfully) full circle pedaling.