I think that showing the cards keeps people interested enough. I don't know about releasing cards a few at a time, because there are balance issues. They have to test interactions with all the new cards, and they want to minimize changing already-released cards. So theoretically, the whole expansion needs to be figured out before any of the cards are released, at which point, they might as well just release the whole expansion...
As far as there not being a lot of deck possibilities, I don't necessarily agree. There are a few more popular archetypes at higher levels, but the fact that more keep showing up says something about the set of unexplored possibilities... Druid is one of the most popular classes atm, and there's no "standard" Druid decklist.
Even if you only look at the decks played by the seeded players at Dreamhack you have:
Token Druid
Taunt Druid
Midrange Druid
Midrange Hunter
Face Hunter
Freeze Mage
Circle Priest
Miracle Rogue
Aggro Rogue
Tempo Shaman
Handlock
Control Warrior
Then there's popular ladder decks:
Zoolock
Face Paladin
Face Warrior
and the occasional:
Aggro Mage
Midrange Mage
Control Paladin
Tempo Rogue
Combo Warrior
Murloc Warlock
plus really gimmicky stuff like Inner Fire Priest, Malygos Druid, or Miracle Shaman...