There's been a big industry focus on test driven development over the past few years.  Many advocates of TDD make it a point to say that TDD is not really about testing, it's about design.  Writing tests before application code forces you to think about the goal(s) of the application code.  For example, what should the input to a certain method be, and what output should it supply?  TDD forces you to think in terms of inputs/outputs and code interaction.  What input should be provided to the callee, what output should be provided to the caller - essentially, how will multiple objects (e.g. classes, methods, etc) interface.  

I've recently started practicing TDD a bit, and have found that instead of always writing unit tests with some type of testing framework (e.g. nUnit), I sometimes create a console application that calls certain methods and reveals the output.  Granted, these are perhaps more "integration" tests than unit tests, as I'm using them to view database contents.  None-the-less, it got me thinking about TDD, and the notion that "Test"-Driven development isn't really about tests.  In my opinion, a more appropriate name might be "Client"-Driven Development.  I'm not referring to the clients that pay your bills, but the client portions of your application (e.g. the UI which calls another application layer).  There are clients at every layer of your app - the UI calls one layer, that layer calls another layer.  Each caller is a client, and when writing tests first, we need to think in terms of the "client", what it should input, and what the result should be.

Isn't CDD essentially the goal of TDD?

Abstraction is a powerful concept in software development.  Abstracting away certain parts of an application can heavily reduce develop time and make for a maintainable solution.  One shining example of this is the multitude of Object-Relational Mappers on the market, which abstract away a large amount of details about database interaction.  However, as with most things in life, abstraction's highest value is realized when used in moderation.  

While picking up ASP.NET MVC, I've started to realize the significant abstraction that asp.net server controls provide.  ASP.NET MVC doesn't have the parallel (at this point) to the server controls of web forms.  There are html helpers, but those are a very minor abstraction in comparison.  

Whether using web forms or ASP.NET MVC, one could argue that there are always abstractions that don't pull their weight.  For example, it's not unheard of for developers to use label controls for every message they put on a page.  The label abstraction doesn't provide anything that an html span tag wouldn't.  As another example, html helpers in ASP.NET MVC don't always provide much bang for the buck.  In some cases, it's harder to learn the syntax of the html helper than to simply write the html itself.  Learning how to write the html always applies to web development, the helpers only apply to ASP.NET MVC.

Abstractions can make us lose sight of how something actually works.  I will certainly admit that asp.net server controls have prevented me from learning the inner workings of some html tags over the years.  At the same time, they have sped up my development in many cases.  Whether you use abstractions or not, you should first understand what each is doing under the covers.  This allows you to determine if it is adding value or just tacking on an extra maintenance burden.