It was a design decision in Gnome 3 (aka Gnome Shell) that involves
some controversy. There are many many threads filled with lots of
debate about it. If you want to know more, mine the mailing list
archives. There are substitute methods to work with the desktop that
require some adaptation of your work-flow (dragging windows the the
edges of the screen, using overview mode with the hot corner,
abandoning a desktop background with stuff on it).

But the long and short of it is, if you want the buttons (and more)
back, just install gnome-tweak-tool and pick the right options to
re-enable them.

/Mike