The Zeroth Law of Usability
Of course by trying to keep my list short, I either left out a things (we'll get to the most important item in a sec) or left a bunch of stuff as a corollary.
0. Function As Intended
There's no point being usable if you aren't useful. This should be obvious but it needs to be said. Blender obeys the zeroth law (in spades!), and it's free and open source, making it sad that it disobeys laws 1 to 11.
Visibility (as stated) implies reduce clutter, organize things sensibly, use visual hierarchies. If I put each of these in as a law in I'd have 50 or more laws and no-one would read to the end of the list. This would make the list less usable.
A classic example of the Visibility law in action is that if you try to say delete something in a good program, the dialog's buttons will be captioned "Delete" and "Cancel" (or similar) rather than "Yes", "No", "Abort", "Retry" or "Fail". Putting the most important information in the first place someone will look is a corollary of the Visibility law.