I tried to explain that in the blog post. Some quick examples: If you back up a Mac app using Dropbox and then restore it, nothing happens when you double-click

I tried to explain that in the blog post. Some quick examples: If you back up a Mac app using Dropbox and then restore it, nothing happens when you double-click on it because the executable file within it wasn't set to be executable. If you restore your "downloaded files" with Dropbox, the "quarantine" metadata won't be there so you won't get the "This is an application downloaded from the Internet" warning. If you restore a "bundle" (e.g. an iPhoto Library) the bundle won't look like a single file anymore -- it'll look like a folder; double-clicking on it would open the folder instead of launching iPhoto. Modification timestamps are all set to the current time, so if you used that information to find things in the past you're out of luck.