|Gnome2 in Gnome3 :P...|
So, if your GPU or PC in general does not support or meet the requirements, then Gnome will fall back to a mode called "fallback-mode", which is basically a redesigned Gnome2+. In this mode, you won't be getting any of those new window based (or the desktop in general) controls or "effects". But, since it does not need a powerful GPU or hardware to render its environment, we can still get a "taste" of some of the Gnom3's new features without having to buy a new GPU with hardware accelerations.
Or, even if you have superior hardware but still would like the "old" Gnome desktop... then you can manually/forcefully "tell" Gnome to always run in the fallback mode.
To do that, first go to "Gnome Control Center" -> "System info" -> "Graphics" and choose Forced Fallback Mode to "ON". See the below screenshot.