Improved support and integration with iCloud is just one of many improvements in Mountain Lion. Improvements include the addition of iCloud document storage, and apps for Mountain Lion will include the ability to either save files and documents to iCloud, or locally on your Mac.
MacRumors points to notes from John Gruber highlighting some of the main differences between Mountain Lion’s iCloud integration and that of OS X 10.7 Lion:
iCloud document storage, and the biggest change to Open and Save dialog boxes in the 28-year history of the Mac. Mac App Store apps effectively have two modes for opening/saving documents: iCloud or the traditional local hierarchical file system. The traditional way is mostly unchanged from Lion (and, really, from all previous versions of Mac OS X). The iCloud way is visually distinctive: it looks like the iPad springboard — linen background, iOS-style one-level-only drag-one-on-top-of-another-to-create-one “folders”. It’s not a replacement of traditional Mac file management and organization. It’s a radically simplified alternative.
iCloud file storage already shows up in TextEdit within Mountain Lion (image credit, Pocket-lint), and will likely appear in many more apps as third-party developers update their apps to support the new iCloud file storage API.
While some have compared the enhanced iCloud support to services like Dropbox, iCloud differs in significant ways – it seemingly requires apps to specifically support the feature, and also only displays files within each app that are relevant to the app itself.