Microsoft wants to leverage their desktop dominance to introduce people to their mobile offerings. The change from start menu to start screen fits right into that strategy. However you can get back the start menu by installing classic shell, pokki etc.
Mostly importantly, tablets and hybrid tablet/laptops are taking off and people want to use the same OS for both. And Microsoft has tried using the Taskbar and Start Menu for tablets before, and it sucked.
Perhaps the price might be a hint that it's worth less than previous versions..