Mobs will no longer randomly spawn if either the block being spawned in or the block above that contains any of the following: buttons, tripwire hooks, pressure plates, levers, redstone torches, redstone repeaters, comparators, redstone dust.

It's not clear whether this was an accidental change with the other changes to mob spawning, and it breaks some things, so filing it to be looked at.

_{Reasoning it might be accidental: in MCP terms, the old logic allowed spawning if !isNormalCube(), which expands to !(isOpaque() && isFullCube() && !canProvidePower()). New logic is !isSolidFullCube() && !canProvidePower(), which expands to !(blocksMovement() && isFullCube()) && !canProvidePower(). They might have meant that to be !(blocksMovement() && isFullCube() && !canProvidePower()) (note moved paren), just swapping the isOpaque() for blocksMovement(), and got confused like I initially did with the double-negative on canProvidePower().}

Code analysis by inasm can be found in this comment.