I assume the intended use case for this spell is to protect an NPC who isn't meant to be a combatant. I say that because otherwise this spell seems completely and utterly useless.

Oof, several significant nerfs from the 2014 version:
1. Specified only affects *damaging* spells rather than *harmful* spells, so no longer helps against things like Charm Person or Hold Person.
2. Spell ends if target casts *any spell* not just spells that affect hostile creatures, so can't cast beneficial spells on allies or self without losing Sanctuary.
3. Spell ends if target does any damage, not just damage to a creature. So cant, say, attack the important macguffin to stop the trap or disrupt the ritual or whatnot without losing Sanctuary.

Seems like the non-damage options of unarmed strike, grapple and shove, would work under Sanctuary. You can give out bardic inspirations or use features that spend those. Characters can pester and annoy by picking pockets or performing the Help action. A barbarian can rage while Sanctuary is active and use every Path of the World Tree subclass feature except Battering Roots. A Battlemaster could transfer an attack to an ally with Commander's Strike. A Psi Warrior could use Protective Field and Telekinetic Movement. Some magic items like Wand of Paralysis wouldn't violate it. 

In the SPECIAL column in the Cleric spell list it does not show "M".

Then when I go into the spell itself it shows "M".

This is true of a number of spells in the special column in spell lists.

Is the special column in the spell list of any use if I have to go into each spell to find out the requirements?

This is still strong if combined with a Healers Kit and the Battle Medic part of the Healer origin feat.

Sanctuary works well on Familiars providing the help action as well as Spirit Guardians casters taking the dodge action to keep concentration on their spell while frying their enemies and that's just a couple uses. 

Maybe I missed something or misunderstood something, but what is the DC an attacker must achieve? Is it a competitive check against the cleric's DC... or against the target's Wisdom...

Quote from PhaxiosNiklos >>

Maybe I missed something or misunderstood something, but what is the DC an attacker must achieve? Is it a competitive check against the cleric's DC... or against the target's Wisdom...

When a spell requires someone to make a saving throw, the DC is always the spellcaster's spell save DC (unless the spell specifically says otherwise, which this one doesn't).

You calculate your spell save DC like this: 8 + (your proficiency bonus) + (your spellcasting ability modifier).