mice and trackpads.

Single clicking can be done by accident very easily.  Nothing dangerous should occur by a single click.  Everything should be reversible.    A double click should not perform an action twice.

hyperlinks, toolbar actions, and buttons are for revertible actions.  A SUBMIT FORM FOR A WEBSITE SHOULD NOT BE A SINGLE CLICK

Buttons for hazardous actions should look differently than a safe action . Perhaps a color change should occur on the first click.

A hazardous action could be defined as anything that could cause data loss. 
Running a program, opening a file,  these are all hazardous.

Dangerous actions explicitly cause data loss.
Saving over a file is dangerous,  (are you sure that is the right file?)
running an unauthenticated program is a dangerous action.

Deleting a file is a dangerous action.
(moving a file to trash could be classified as hazardous)
    


Taps frequently occur without the user knowing.

Tap-drags are pure evil.

I like how Microsoft has a default action of copy when dragging files between volumes.   I like how they move by default when dragging within one volume.

I also like how Dolphin asks me what I want to do.   but I don't like how it gets in my way.

I would like to have an informational dialog box show up, saying "move to" or "copy to" when I approach my target. 

Holding down both buttons on a drag should cancel the drag.
Right click dragging should bring up the questionbox. (on drop)

(Yet another reason why Macintosh's are horrible)