Some things I like about the classic style:
* The title bar of the active window is shaded in the accent colour, those of inactive windows are not. This makes it clear at a glance which window I'm typing into.
* Unless a developer goes out of their way to mess with components, a button always looks like a button and things that look like buttons are always buttons.
* In the control panel, you can *easily* customise the fonts and colours for things (like picking your own colours for both active and inactive title bars). Yes you can make completely horrible setups this way, or set your co-worker's PC to comic sans as a joke, but someone with dyslexia, colour blindness or other disabilities can also easily set the UI to exactly what works for them. "Themes" in later versions are both less flexible and require someone with more time and technical skills to create them in the first place.