Some users still find confusing setting opacity to whole layer.
When they change opacity value in Transparency palette with no art selected, it does nothing or (if 'New Art has Basic Appearance' is unticked in Appearance palette options) applied to new objects.

The way transparency is set for a layer in AI is not that obvious, compared to Photoshop's way, and require from user knowledge of working with appearance.

I propose this:
1. Make a shortcut for 'Target Current Layer' command, which is not anywhere now (or even 'Target Selected', which will target current layer, if nothing is selected)
2. Add 'Target Layer' button to Transparency palette, when nothing is selected, in the empty area, where selected artwork thumb is usually displayed

These will give users ability to quickly target layer and set it's opacity, both mouse and keyboard oriented.