Many Mac fanatics become very disgruntled when Apple introduces a new custom look for a certain app or suite of apps. They argue that there should be consistency throughout every Mac OS X interface. Down to the little details such as colors and font sizes, these people won't stop at any cost to argue their point. Some go as far as to make modifications for apps (here's Aqua in iTunes 7, which looks just awful).

Are they right or are they wrong? Well, they're both.

User interface design consistency is important, in a miniscule way. You do want to make things intuitive and easy to use, and the best way to do that is by copying existing layouts and techniques (as described in the Aqua Human Interface Guidelines). Many developers do this (though some don't, and it shows). The best way is to have consistency where consistency is needed. Alert dialogs and print windows are very good examples of this. The "two column" view seen in iLife apps is another great example. Most new Mac users can quickly figure out how to use these apps after simply using one (usually iTunes). Some smaller examples are the close/minimize/expand widgets on every window and the menubar. These consistency decisions are important.

So why are they wrong? Because, in reality, designs can vary. They do not need to be the same. In fact, being unique and different is a really good thing. It makes an application easily distinguishable from others. A good example is GarageBand - by using it's "Amp" graphical interface, you can quickly identify that the app is related to music and that it's very different from other applications. The very first iTunes was another great example. Instead of being bland and boring, it used the "jukebox" metaphor to help distinguish it from other system apps.

iTunes 1.0 had a great jukebox look

GarageBand's style is cool and amp-like

What does it matter if a button is a different color or different font? Sure, it might not look as good (there's a really good chance of this, but that's why we have designers) but to new users it's not a big deal. If it looks like a button, they'll push it.

This is why iTunes, for example, has all of those weird new looking controls. The scroll bars are different colored - so what? They are still extremely similar to the Mac OS X scroll bars. It is definitely not a "sign of things to come" as some people predict, just like the GarageBand GUI was not a sign of things to come, and just like the Aperture GUI was not a sign of things to come. If future apps do have similar looking controls and elements, it is not because Apple had planned this out, it is because they made a later decision to make them look similar. The (at one time very scary) Mail in Tiger look has been adopted widely across Mac OS X. If Apple had planned this out, many apps would have had a similar style in the first release of Tiger.

Inconsistency seems to work well for third party developers as well. Take Delicious Library, an app that may not have been nearly as popular without its amazing "shelf" look. (at one point, the entire app was going to have a wood style, which might not have looked too bad).

Delicious Library, with wooden background

I am not sure this goes for anyone else, but I would probably get bored if every app looked the same and had no innovative graphical decisions. I might even get confused if interfaces do not work the way I think they should. So in a way, inconsistency is consistent, and that's a good thing.