I have a Windows 7 pc and a Windows 8 laptop.
Both allow mw to use icloud and photo stream with out having to add any extra apps.
I upgraded my imac to mountain lion doing a fresh install and then installed ilife and updated it as far as Apple would update it.
I try to turn on photo stream it is grayed out and says:
Shared Photo Streams allow you to share selected photos with people you choose. This Photo Stream feature requires OS X Mountain Lion v10.8.2 (or later) and iPhoto 9.4 (or later) or Aperture 3.4 (or later).
It says I have to buy the newer version.
My point is that I do it for free on my Windows computers and have to pay for something on my MAC.