As of Patch 5.4.0, Holy Word: Sanctuary functions as a 'smart heal', healing the 6 most injured friendly targets within range. This appears to contradict the ability's tooltip, which may simply not have been changed to reflect its new functionality.
Patch 6.0.2(2014-10-14): Now heals for 60% more than before.
Now costs 2.5% base mana (down from 3.8% base mana).
Patch 5.4.0(2013-09-10): Mana cost reduced to 3.8% (down from 6.3%), and now functions like a smart heal, healing the 6 most injured friendly targets within range, and minor guardians are no longer targeted (Wild Imps, Bloodworms, Snakes from Snake Traps, etc.). There has been no change to the total amount of healing granted by the ability.
Hotfix (2011-04-27):Holy Word: Sanctuary should now properly benefit from Echo of Light.
Patch 4.1.0(2011-04-26): Holy Word: Sanctuary healing done has been increased by 35%. In addition, it has a new spell effect.
Patch 4.0.3a(2010-11-23): Holy Word: Sanctuary: now has a diminished effect when healing more than 6 players at once.