Mastery or Haste is personal preference. Both are very good stats and you can pick the one you prefer (Haste is slightly better in 10man, Mastery in 25man). Another possibility is striking a balance between the two.
Crit isn't a bad choice in and of itself. It's bad because all the other available stats (Mastery, Spirit, Haste) are so much better. Crit provides throughput, but is unpredictable, and can't be relied on. Healers love predictability .
As above poster said, haste and mastery are my top stats as holy, but it's a matter of choice. I gemmed and reforged enough to get the extra renew tick - comfortably - in heroic gear before I started stacking mastery. while mastery is slightly less effective in 5-man dungeons or tank healing, it shines in 25-man, where you will never have enough mana to top off.
I would pick up crit on arcanums and inscriptions but I wouldn't gem or enchant it if I could avoid it, it simply because intellect would always be a better choice for gems, as it provides a constant PREDICTABLE (we love that) stream of spellpower and crit. After that, mastery and haste are at your discretion.
A look at holy priests in top raiding guilds proves this, as Paragon's top Holy has stacked haste to the gills, while the top holy in Stars has stacked mastery. Of course, those guilds' players optimize their builds to complement each other, so each may have another holy with the opposite setup. Just saying, both are viable.
NOTE: Raiding, my lock still puts Dark Intent on me. Any haste boost is welcome when you're spamming low-output Heal spels or need to blast out a prayer of healing (as raiding we holy priests generally pull double duty, healing an off-tank and the raid when we can if two-healing), and with renew ticks, circle of healing and prayer of healing, there's always a crit for him to feed off of resulting in near 100% uptime for both of us on the increased spelllpower. This works even better now that Surge of Light can crit.