The Terraria 1.2 update introduced that make generated worlds varied not only in layout, but in availability of various features as well. What is the fastest way to get a world that has the features I need? Is there a program that can help goad Terraria to generate a world that fits my criteria, or perhaps a tool that will let me quickly tell if a generated world has the features I have in mind?
Very simply: If you want to have either Crimson or Corruption, look for the text during world generation that says "Making World Bloody".
If you see this, you've got a world with Crimson, not Corruption, and can quickly choose whether to keep or discard this world.
Similar text clues might also hint at what special features your world will contain, but beyond that, if you want to just 'get' what you're after, you'd be better off just looking for and downloading a world file that meets your needs.
EDIT: As of 1.3, you can tell if the world is corruption or crimson by looking at the texture of the loading bar.