Prioritize lowest health %, but if a tank is low too, prioritize him first. ( if there are multiple tanks, then prioritize the lowest one )
Every tank will have different healing settings. ( spells and % )
Check threat, so if a tank loses his aggro for X milliseconds, you can set a focus heal on the non tank player with the highest threat. ( shield, bubble, barkskin, quick heal etc. )
Pre hots and shields, if there are encounters of a boss, which will give players a decent amount of damage. ( debuffs, or a cast of the boss )
Check debuffs of all players and if you can dispell it. ( prioritize tank ofc. and avoid overheal, if there's a 100% healing reduced debuff, which can't get removed by overheal )
At some boss encounters, a certain healer will have to use his cooldown, which will get choosed before. ( example: At the 5th charge of the boss i have to use tranquility )
Different settings and spells at high and low mana %.
Possibility of only healing certain groups and players, in a raid. ( if they're not alive anymore, switch to the other groups )