The toggle feature of the Comparator block is not working. I have tested any direction, the signal strength of the output is always the same as the strength of the input. It doesn't subtract either.
The one-clock in the wkbde.png image seems to work, though.
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Toggling does not increase signal on comparator.

Toggled state is now "subtraction" mode. In this mode, O=A-B. You're giving or A but no B, and A-0 == A.
For me it doesn't subtract either. I have input A which is 14 and input B which is 12. The output is always 14 even when I toggle the comparator. It should be 2 when the comparator is toggled (subtracted: Output = A - B)
@Robin: Yes, I can confirm that.
This seems to work though.

B needs to be provided with a type of diode (repeater or comparator).
So the toggling is useless and only the block type of the input changes the function?
How will that work as a repeater or comparator will always provide full redstone output? I haven't worked out a single layout where switching the torch on or off would change anything output-wise.

No.
A == Input from front.
B == Strongest input from side given through another comparator.
O == Output from back.
Repeat mode (Default):
O = A
Subtract mode (Right click):
O = B - A
You can only provide B using another comparator pointed towards the side of this comparator, much like you would lock a repeater.

See http://dinnerbone.com/media/uploads/2013-01/screenshots/Minecraft_2013-01-03_21-19-52.png
A comparator will only ever provide full output if you're giving it full input.
Ah, I exchanged B and A. So it's like an analogue inverter. It would be nice if you didn't need a repeater and could use different values for B.

You can use whatever value for B you want. Use a comparator instead of a repeater so that you keep the signal.
Thank you! Now I got it 🙂
So toggling the torch only affects the output if another comparator points at the comparator? That doesn't seem to be very intuitive. Redstone stuff is complicated enough already. Why does the B signal need a comparator there? Why can't you just point redstone dust at it which has different signal strengths on it's own?
@Jonathan Haas: That's what I'm thinking now. But it's just a snapshot, so ... 😃