No, they don't have to be flat. There are two items that hinder it. One is that winamp has a fixed size, modern layout systems have to support a multitude of

No, they don't have to be flat. There are two items that hinder it. One is that winamp has a fixed size, modern layout systems have to support a multitude of screen sizes, which is a hard problem. Throw on some nice 3d texture as a full skin and getting that to work is much more difficult. It's not impossible, but some of the items on the skins would be very hard. The other big one is accessibility. People with poor eyesight can struggle when the system has low contrast. 3d buttons or adding depth to controls and the like are pretty simple and don't have any major impediments that I'm aware of. I often will accent my GUI designs with a light touch of depth. I can't tell you for sure as to the evolution of the systems, I design modern systems and while I did toy with GUIs back in that day I didn't do major systems then - I was a kid. Aesthetics takes a major backseat to usability in my work. Though I'd say that some of today's look and style is much like clothing. It's a style of the day.