|Curved and Enjoyful
The Mac was designed to be a "nice" guy and make computing accessible to the masses.
Friendly, funny UI elements and icons combined with curved designs and simple concepts, as they have been introduced in 1984, made the Mac the computer "for the rest of us".
The system gives you a lot of freedom that you can use to turn the system into your personal Mac. However, it was a single user idea, which drove the design and with all the "freedom" it became also very easy to clutter your virtual desktop in the same way like you real one.
It should be fun to work with because it is your personal computer, your Macintosh.
|Straight and Productive
Scientists and business professionals in a networked environment were the target audience of NeXT.
The "clean and straight" design philosophy started with the NeXT logo, the original NeXTcubes highly symmetrical outlines and reached even down to the pure object-oriented programming interfaces.
Its user interface was much more focused on functionality, network awareness and a clean (some might say "cold but colorful") appearance. This should not be misinterpreted as if Keith Ohlfs artwork was not nice - it still is the most exciting piece of icon and UI design to many people - but it followed a less "playful" approach then the Mac.
Productivity in a visually pleasing and clearly organized environment is what might describe OPENSTEP.
|Curved and Productive
With OS X Apple is aiming at novices and professionals, old Mac users and people who have never used a Mac (or any computer) before ... and sure ... the internet plays a central role as well.
The design must be sophisticated, colorful and fresh so that it attracts new customers easily and underlines the power of the entire system. Mac users must feel at home as much as possible but new features need to be included to support the new capabilities of the OS and to give powerusers easy access to them.
To some degree it evolves along the same lines like the Apple logo. On the one hand it was simplified down to its pure shape which made it more recognizable and versatile. On the other hand it now comes in many different colors with a fancy translucent shading giving it more details then it had before.