Soul Sand is 1 pixel shorter than most blocks, and is the same height as tilled dirt. This means one cannot climb from soul sand onto a slab without jumping (due to the greatest height you can walk up being 8 pixels). These can be used to make one way doors.
Soul Sand also slows the players movement speed when walked over. This can be used to slow down mobs or other players when they attempt to access your base. The slowing effect of Soul Sand can be amplified by placing Ice below it.