John Gruber mentioned in passing that people are confused about when to double-click and when not to. It's true, but that doesn't just apply to users. I've seen many application developers not knowing (or simply not caring) about when to use a double-click, and when not to.
The simple matter of the fact is: Double-clicks are a shortcut.
Look at the Finder: A single click selects an object. A double click opens it. A double click here is simply a shortcut for a single click ("select this item") plus the most common menu item used on this item ("File" -> "Open").
Many users are simply never taught that this is why to double-click. Many think "Files are always double-clicked".
In the dock, you can't select an item. So, a single click already triggers an action.
I won't count minimizing windows by double-click here. Why? Because it's actually a historic feature. Back in System 7, you couldn't minimize windows. There was a title bar with a close button and a zoom button. Someone wrote a nice extension called "WindowShade" that rolled up a window into a title bar. Since they couldn't add a widget to every window, and single clicks already dragged the window, they just decided to use a double click. When that extension got rolled into the system with System 7.5, the shortcut stayed, and never got removed, even after MacOS 8 added the collapse box widget.
I don't know why this feature even still is in OS X. We have a "minimize" widget taking up valuable screen real estate. Why even leave something like that in? So many newbies accidentally trigger this and wonder where their window has gone.