Zenyatta’s Orb of Discord and Reinhardt’s shield

Composed with assistance from this Source:

The Barrier does NOT block the following:

• Melee attacks. Reinhardt and his shield will both take full damage from a single melee attack.
• Sombra's EMP will destroy his shield, and hack everything that was behind it.
• Orisa's halt ability will pass through it, however it will not suck his allies from behind a barrier if it detonates on the other side.
• Some channeled weapons, including: Winston's Tesla Cannon, but Symmetra's Photon Projector beam will be blocked.
• Reinhardt's Fire Strike and Symmetra's Photon Orbs.
• Hanzo's Dragonstrike
• Junkrat's Rip Tire

Fire Strike and Photon Orbs can actually pass through and damage everything in its path, including Reinhardt, his barrier and anyone else behind Reinhardt.

Do also note that Reinhardt's shield does not block ally projectiles. This means you should combo with a Bastion or any other front line assault that will make best use of your shield.

Allied projectiles will go through Reinhardt's shield without difficulty. There is no friendly fire in Overwatch, but your own splash damage abilities can hurt yourself (Junkrat and Pharah most notably).

Also note that there are a couple of enemy projectiles that penetrate his shield, see this question for a complete list: Does Reinhardt's shield block all abilities?

The actual players can also pass through the shield without difficulty, both enemies and allies.