Are grayscale buttons with icons usable and effective?

enter image description here

The image here is

  1. old gmail
  2. new gmail, when no mail is selected
  3. new gmail, when mail is selected
  4. new delicious

To me, the buttons are unlabeled and tough to discover.

I don't believe the icons do a good enough job to make labels redundant. Also I noticed:

  1. the Gmail toolbar changes entirely when a mail is selected, whereas the old toolbar was (reassuringly) constant
  2. the delicious edit and share buttons (encircled) are tough to discover and don't have on-hover tooltips

My first reaction to this was "backward step", since they made the options tougher to discover.

But other sites seem to be converging to this style, a design language that tries to be as sparse as possible. It's definitely good for iPads and netbooks with extremely high resolutions.

Is such a design language worth it, or too tough to decipher?

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