The past few large-biomes worlds I've created in the Minecraft Snapshots seem to be almost devoid of iron, halting in-game progression early on.  Typically I will have mined a stack of copper and coal and full chest of other stone before finding any iron at all.

Stats from most recent world:

Seed: 1381865077

2196 Stone Mined

102 Copper mined

8 Gold Mined

6 Iron Ore Mined

3 Deep Slate Iron Ore Mined

43 Stone Picks Crafted