Yosemite 10.10 the dock is corrupt
 

The dock has a white solid background. On the new beta driver. Question, how is there no release driver for a released OS?

More in general I think that the "vibrancy" effects are not available on non accelerated screens.
This is done when the items are composited, it's an effect that is probably not yet supported by the software renderer.

Still, that does not explain why the dock is so unbelievably slow, leading to the transient corruption that I can see.
Looking at the Console it's obvious that the WindowServer has no clue about how to handle the screen and likely falls in some error recovery mode every single time the Dock is drawn.

We have fed back all of this to Apple a couple of months ago. Let's see how it goes.

In the meanwhile a workaround is to go to Preferences->Accessibility->Display and enable "Reduce transparency". This makes all screens to drop the new effects and speeds up rendering (a bit).

On a personal note, I'd like to have the translucent Dock but I agree with John Siracusa and find the vibrancy on normal windows outright disturbing. I immediately flicked the Accessibility option anyway on my system, before connecting any external screens.
It's a matter of taste though and I would like to see the new effects working well on all screens.

Cheers,
Carlo

Carlo, did not work, makes the white square around the dock lighter, that's about it.