Both anime are extremely well done harems. DxD and Shuffle both balance plot and harem aspects really well. The size of the main character group is about the same or equal. Both shows have beings who can use supernatural powers without everyone needing weapons; many characters have sacred gears in DxD, but not everyone and even so, they're more like relics than weapons. Both shows have an amazing cast of females and a unique complimentary protagonist; they're not dull and they fend for themselves a bit. Both harems have a main love interest as the story progresses.