Is it the thickness of the fabric? Construction? Or something else?
I'm experimenting with Wanderer. The construction is different from a heel flap so I'm hoping my husband has less issues pulling it on his foot. He wants the sole to be double thickness which is one of the ways I'm playing. The other is doing it as two pieces with the sole knitted to the top a la knitting an edging on to a shawl.
But if the only difference between a sock and a slipper is the fabric thickness then there are plenty of other ways to do this. This comes to mind, for one.