The AppleWorks UI was kind of an incoherent mess, stuck halfway between the Platinum and Aqua UIs. You can actually see elements of this in the screenshot -- the ruler in the word processor window is a relic of Platinum, with gray controls and bitmapped text, whereas the window frame and menu bars are more of an Aqua thing. (Except for the AppleScript menu, which is a Platinum-era icon again.) Then there's the "Button Bar", which was an entirely idiosyncratic element added to the OS X version of AppleWorks...

Much of this is due to the history of AppleWorks -- it was a Carbon conversion of ClarisWorks, which dated back to 1991. Parts of the UI -- like that ruler -- are essentially unchanged from version 1.0.

For the most part, that looks like it's after the pinstripes were toned down, but before the brushed metal fad took hold. So there's plenty of use of color and enough depth to make it easy to distinguish between UI elements. But as noted by others, there are some legacy holdovers on display, too.

It's probably one of the most "timeless" looks ever used by OS X.

I find flat more aesthetically pleasing but often harder to use, and I've seen data to back that up.

Problem is that people tend to confuse the two and fool themselves into thinking pretty equals usable. Glossy vs matte screens are the classic example. Glossy looks pretty but has way more issues with glare.

I am generally confounded by the flatness of Window 10. The ability to see which window is on top is gone!