For starters, the article seems to be mashing up 3 concepts:
- 3D effects vs. no effects (iOS's 'bubbles and drop shadows' vs. Win8's 'flat everywhere' aesthetics)
- 2D vs. 3D interfaces (Your standard computer OS UI vs. your favorite 3D FPS)
- 2D vs. 3D screens (iPhone vs. Nintendo 3DS)
All interesting topics, but they are all really separate topics and not mutually exclusive. That said...
Q. With even more powerful platform coming in, why is there a paradigm shift now, as the article suggest ?
In terms of styling (first bullet point) it's really just an aesthetic trend. Flat is 'in' right now.
In terms of true 3D (be it virtual 3D or the screen itself) there is no real paradigm shift. We've always used 2D interfaces for the most part. We've played with 3D off and on (VRML, 3DS, Xbox Kinect) and they all work to some degree for specific tasks, but, in general, no one 'wants' a 3D interface to work on a Word Doc or read StackExchange.
Q. Do these factors actually deserve equal importance, or there is an ordering in them ?
I'd say Performance and Look and Feel are merely aspects of Usability in general. So they are all equally important.
Q. Did we wrongly considered 2D to 3D as evolution ? Is , 'simpler', the actual evolution in UIs ?
Again, we've played with true 3D interfaces but I don't think we've found them to add anything of worth to the overall experience most of the time. For gaming, sure (Halo!). For movies, of course (Iron Man!), but for most general day-to-day tasks that users have to perform on their digital device, true 3D is likely going to get in the way more often than it will help.
As for the aesthetic styling of 2D interfaces to look more 3D (what some people like to call skeuomorphism--though I'd argue that's not the proper use of the term)--such as drop shadows, subtle gradients, light effects, etc--those are basic tools in any UI designer's tool box. They've always been there, and always will, but stylistic trends will dictate to what extent they are being used at any given time. The past decade was XP and OSX embracing bubbles, gradients, reflections, and shadows. The current decade appears to be heading in the Win8/Google direction of flat colors, no shadows, etc.