Tim Ottinger has a nice post on how TDD challenges your old coding conventions. Some of the changes TDD encourage include:

• More interfaces
• Singletons/static methods considered harmful (or at least hard to test)
• Replacing private members with appropriate interfaces.
• More factories, less calls to concrete constructors
• Interface design by actual use (rather than anticipated use)
• Dependency avoidance.

To quote Tim:

For the most part, TDD forces you to start doing the things you always should have done.