The iCloud folder that works exactly like dropbox is not in the build of Mavericks given at WWDC. Apple said it will be available to the developers in a future build.
Yes there will be a publicly visible iCloud folder that when documents are placed within will be openable by any application. The private folders for each application will be mostly hidden in the finder.
This is basically a dropbox killer with much better CPU usage, and much better bandwidth management.