I did a standard Win8 install plus Office 2013, web browsing (Firefox beta, Firefox Nightlies, Chrome, plus the built-in IE), plus a few small apps from the new Windows Store (Skype, Google, Solitaire, Minesweeper, Netflix). The whole thing fit and ran on a 25 GB partition without a page file, but it was filled to the top. To Win8's credit, it handled having <100 MB (yes, megabytes) of available HD space really well.
Basic point is that 40 GB is enough for a basic functional install even without cutting back, but you won't have a ton of room left over for media or other stuff. Basic steps like turning off hibernation, turning off system restore, and reducing page file size will help, but I haven't seen much about reducing the install size of Win8 yet. I'm sure people are working on it.