Healer substat optimization is a bit tricky.

If you're looking at damage output and ONLY damage it goes like Direct Hit > Det >= Crit > Spell Speed. The thing with DH is, you need a fairly huge investment to make it worth it (full Heaven's Eye VI melds, all slots) and it doesn't affect your healing output, unlike Crit and Det. Crit/Det are more or less equal - I think Det slighly outweights Crit as of now, but that's because healers can't get a good amount of Crit in this patch - Crit will very likely be the best stat when ilvl goes beyond 340. I'm personally not a fan of Spell Speed at all, the returns are too weak and most pieces that are Spell Speed centered aren't bis.

I believe the goal of every healer shoul be to minimize Piety melds as much as possible - and also shoot for gear that have no/low Piety, reason being that it only acts as a MP cushion, bringing no benefit to either your damage or healing.