So... crafting a constructed deck... where do I go from here? I have about 500 dust lying around doing not much, and a half-dozen epics lying around (including 2 BGH, a Faceless, Shadowform, Lay on Hands)... not enough to construct a truly "good" deck yet but perhaps enough to construct an interesting one. Should I craft an epic, saying a second Faceless or Shadowform? Or craft some extra rares, a second Defender, a second Azure Drake, a second Shadow Madness?
Is it crazy that I'm leaning toward playing a Shadow Priest? Is that completely outside the scope of the meta?
I'd just go with what you have, and save your dust for when you're missing only 2-3 cards for a deck. Like theory says, you'll open a lot of the cards eventually if you keep playing -- creating a card and then opening it next pack is a pretty bad feeling.
Crafting is tricky. What does it mean to be 2-3 cards from a deck? If you're playing at low ranks, you don't need to match the decklists of the legend rank decks people post since the meta is going to be different. At high ranks, it seems Priest needs Auchenai Soulpriest + Circle of Healing, as it's the best way to deal with concealed Auctioneer. But if you don't see any Miracle Rogues, you can get your board clearing from some combination of Novas, Pyromancer, and Shadow Madness. Similarly, Holy Fire is a necessity for a high level Priest deck, but if you just run Argent Commanders instead, the deck is still playable at some level. But that's something you have to figure out -- or take a guess and just go for it. There are a few cards you should be more willing to craft because they can be used in a lot of decks and are not that easy to replace:
Azure Drake, Argent Commander, Sunfury Protector, Earthen Ring Farseer, Argent Squire, Wild Pyromancer, Knife Juggler, Leeroy (if you're going for a legendary)
If you need to fill in a deck with free stuff, your best friends are:
Sen'jin, Yeti, Ooze, Boulderfist Ogre, Gnomish Inventor, Bluegill/Wolfrider (as pseudo-removal), Cleric, Raptor, Grizzly