The difference was that iDisk didn't cache its stuff on your computer so you can access it and manipulate it offline then have it sync itself. Instead it was more like an FTP server where you were always working across the network. Was so slow and buggy. The new paradigm of having all the files on your computer and syncing them to the cloud is much more reliable. And iCloud even does keep your stuff on your computer, but you can't really put files there yourself as it's not designed to detect these changes and sync right away. For instance, I would put a file in my iCloud folder and it wouldn't sync until I was on the other machine and did something unrelated that also happened to need iCloud in order to force it to sync. So they just need to flesh it out a bit, make it so the folder is in your Home instead of Library, and sanitize it so it isn't full of randomly named folders for each app and instead lets you use a normal structure. (Right now it only does a single depth folder structure, even though you can manually add another depth, it won't be recognized by the app.)
I mean I have 4.5GB that I don't even use right now. If I could use it for other files, I'd actually probably buy an account and use my storage.