Dan Lurie from The Unofficial Apple Weblog wrote iTunes 7 UI: the Bad and the Ugly:
Regardless of your opinion toward the new look, you'll probably agree that when it comes to an interface we use day in and day out, consistency is not just a nice thing to have, but crucial to keeping us users sane when switching between applications.
Consistency in the aesthetic sense, that is. Dan Lurie writes about aesthetic consistency when he believes that he writes about consistency in the usability sense. In reality, it is interface behaviour that is important. If consistency in regard to aesthetics would be so important we wouldn't be able to browse the web at all considering how many different visual designs are out there. Neither would Exposé be of much use if all windows looked exactly the same.
In reality, visual clues such as different textures and hues might be what keeps things separated in our cognition. What is important is still partly an aesthetic issue: Cognitive Scientist and also former Apple Employee Donald Norman wrote in his book Emotional Design that Attractive Things Work Better:
The psychologist Alice Isen and her colleagues have shown that being happy broadens the thought processes and facilitates creative thinking. Isen discovered that when people were asked to solve difficult problems, ones that required unusual "out of the box" thinking, they did much better when they had just been given a small gift â not much of a gift, but enough to make them feel good.
In other words: things that look and feel better work better. But if that is true, why do I write this article at all? It is because the concept design is often associated with only the visual qualities of software. And if aesthetics did not play a great role in the issue we would not be talking about the Look and Feel qualities of e.g. Macintosh software. Even Sun Microsystems has articles on Look and feel on the Mac.
So. But what if the new iTunes interface does not make the users feel good? How can we know what to do about it? Especially if an equal number of users like the new better than the last. Dan Lurie continues:
The other possible explanation of the new design is that Apple has stopped caring so much about design; and this scares the crap out of me. Since the beginning Apple has placed huge importance on good user interaction in all facets of their product line.
No, it is rather so that visual and graphic design does not equal usability. Design is actually more behaviour than pure graphics. David K. Every of iGeek tangents the issue in his article on Copying Files and interface behavior:
Now the first thing about consistency is that it is consistent; meaning it works the same. [...] Think of all the flavors of Windows and all the patches. Things change between each of them, including almost all the behaviors I document. It is very troubling because any time you say, "it works like this" you are wrong, because it only works like that on some versions or in some modes.
It is called interaction, not intervision for a reason. When we use technology we interact with it – we are not part of a linear series of events. That would be like watching a movie. What we do is more akin to communication between the technology at hand and the user, a concept that would also be true in the act of designing the software. This is what is often named distributed cognition when technology and user together achieve more when they work in tandem than they would have if they did not work together.
Consider simple calculators, they all may look different but all work by the same rules. The user also has knowledge on how to do arithmetics and all the mathematical rules the calculator also might be programmed with, however he/she also has a limited time and mental capabilities. Even though he/she knows exactly how to add two large numbers it gets strenuous in the long run. Therefore it is better to use some kind of help to complete the task rather than go through with the task by hand.
Of course things should look good, however calling foul on interaction design teams because you don't like the aesthetics of the software is pointless because the main objective of interaction design is to make interaction as optimal and easy as possible.