Minecraft wiki entry for Opacity, associated text: Full placement rules can be found at Opacity/Placement
Mobs cannot spawn on transparent blocks, nor spawn inside opaque blocks. However, they can spawn inside a non-solid transparent block, such as flowers.
Chests cannot be opened if there is an opaque block on top of them. They are not affected by transparent blocks.
Water that is completely surrounded by water or opaque blocks does not have a current. However, if one of the surrounding blocks is transparent, the water shows a downward current on its side, like a waterfall, although swim speed through that water remains unchanged.
Redstone behavior (see Redstone circuit and its subpages for details):
Opaque blocks overhead can prevent redstone wires from connecting to each other.
Only opaque blocks can be powered by power components or transmission components (see list of redstone components).
If a transparent block can carry redstone wire, it acts as a diode, carrying power upward but not downward.
Tree leaves can overwrite transparent blocks when they grow, but they do not overwrite opaque blocks.
Opacity affects door orientation when placed. The door's hinge, by default, is on the half of the block where the player was pointing, but if there is an opaque wall on one side and not the other, the hinge appears on the side with the opaque wall, regardless of where the player was pointing.
Ghast fireball explosions start fires only on opaque blocks.
Particles that are affected by gravity (e.g. slime particles) fall through non-solid blocks, but stop on opaque blocks.
Bats cannot hang on transparent blocks.