I was excited to see that when I launched Minecraft today, I saw 1.5 had been released. I was thrilled, so I created a survival world and checked out the improved engine and some other features. After a few minutes, I noticed something as I was chopping down trees: the block outline and the breaking overlay was showing over the particles!
The problem is that the block outline (the 1 pixel thick, black line that appears around a block when your cross-hair is over it) and the breaking overlay (the image that is shown on top of the block to make it look as if it is 'cracking') is show on top of the particles coming from the block.
I've attached some screenshots of this happening. Notice that the large particle in the top-middle of the picture has both the block outline and the breaking overlay shown on top of it, yet it is actually meant to be on top of these.