David at 37signals writes that the days of consistency are over (via HMK’s Spurious Thoughts):

“With the introduction of iTunes 5, Apple removed all and any doubts. The days of consistency for user interfaces in the look and feel department are over. The rise of the web killed it and Apple is riding this new wave with no regards for the old HIC guard.”

When Bret Simister and I gave a talk last year at BayCHI about Laszlo’s Cinematic User Experience, a member of the audience asked what we could do to ensure consistency in this new space. We held the controversial stance that the genie was already out of the bottle. Since web sites are not consistent in look, web applications need not be. Establishing identity is a crucial part of web design and it is our responsibility to enable web applications to have a unique style.

If you closely compare iTunes 4 vs. iTunes 5 you can see that many of the design details as different, but one could hardly argue that the average human would not be able to understand the interface because of those differences. I would even put forth that the average human would not even notice the visual differences: brushed metal vs. gradient, the corner radius and margin size.

Visual changes themselves do not compromise usability. More significant is consistency in behavior. When we provide rollover feedback on a small square, rectangular or oblong area of the screen, it is readily recognized as a button which may be clicked. Scrollbars are as easily identified, whether the arrows are brief chevrons or filled triangles, despite alternate layout or color. There are a dozen or so familiar knobs and dials easily recognized in your typical application by their general shape or shadow and how they react to you. The exact pixel dimension, color or symbolic representation is less important than the way they feel.