Well the iCloud Document Storage, at least for my understanding, must be integrated into the apps you use. Thus our free choice for the apps to use is limited to those with that implementation. Apple said they want to get rid of the file system. This can make things more complicated than improve. If you have one folder for clients xyz and you want to store all related docs, e.g. Spreadsheets, documents, presentations, photos, PDFs, then this won't be possible. Your files will be cluttered among all the related apps. There obviously won't be one central place anymore from where you can see all the files related to a certain client. No filesystem = no file browser.
This concerns me quite a bit and Dropbox might still be the ultimate cloud storage to use for that.
As much as I love Apple as much I hate that they want to tell me the way I have to work. They should leave that choice up to us and thus also implement dropbox into iworks for example.