Areas covered in bottom half slabs will be unable to spawn mobs, no matter the light level, although double slabs, top half slabs and upside down stairs are still spawnable. Mobs will also not be able to spawn on top of transparent blocks, such as glass, or partially transparent blocks, such as leaves.

Can mobs spawn on slabs?

Solution: Mobs cannot spawn on normal, right-side-up single slabs (the ones occupying the lower half of the block), as they're considered transparent blocks. They can spawn on upside-down single slabs (as of the 1.5 update) and double-slabs (which are full, opaque blocks).

What blocks can mobs not spawn on?

Most mobs never spawn on transparent blocks, in water (except fish, dolphins, turtles, and other aquatic creatures), in lava (except for striders), on bedrock, or on blocks less than a full block tall (such as slabs placed on the bottom half).