The 1.2 update for Terraria went live a couple of hours ago, and I've heard worlds generated after the update will have some new unique features such as ores being replaced and the possibility of rare biomes and structures. What are the different features that a world may or may not have after generation and what do they mean for the player?
All metal ores have a possible replacement.
The differences between equipment made with replaced metals are not entirely cosmetic, but they are small. The new replacement metals are in general stronger than the materials you may already be familiar with. Ore replacements are total - a world will not be generated with both gold and platinum. However, it is possible to obtain any pre-hard mode ore through the Extractinator. All crafting recipes that use metal objects have versions for replacement metals, e.g. a 1 second timer may be made from either a gold or platinum watch.
There is also a possible replacement of Corruption with Crimson; generation of the Crimson replaces all corruption blocks and monster spawns, demon altars, shadow orbs, the Eater of Worlds, and determines whether demonite is replaced by crimtane. During world generation, the text "Making the world bloody" indicates generation of Crimson. Crimson terrain is characterized by different shapes than the corruption. Crimson spreads through Sunflowers even before hard mode. Trees harvested in the corruption now yield the new block ebonwood, whereas crimson trees yield shadewood.
Worlds may or may not spawn one or more large trees, which in turn may or may not have root structures. If a root structure exists, it will have a room containing 2 living wood chairs, 1 living wood table, and a living wood chest containing an item that can only be found in such chests. An example of such an item is the wood wand, which places wood blocks as living wood, allowing you to expand the tree. This room will need only a light source to become suitable NPC housing.
Worlds may or may not generate a pyramid in one of its deserts. Pyramids have an entrance near the top into a single zig-zag hallway, punctuated not too far in by a chamber with a golden chest and loose coins. The chest will contain one item not found outside pyramids. An example is the sandstorm in a bottle, a superior alternative to the cloud in a bottle.