More Strategy
Some argue the Humans require much more strategy than the Orc player. The Orc player often needs and builds only Bloodlusted Ogres, while the Human player has and needs all sort of spell combinations, units and strategies to counter the Orc threat. A Human victory over Orcs that reached the Fortress/Bloodlust stage is often something to brag about because of the skill level and strategy required.

More Damaging Attacks
Humans can clear out any Death Knights with Exorcism, and can use Mages and Invisibility to destroy all of the Orcs mining operations. The Human player can then slow enemy Bloodlusted Ogres, and catch them without Bloodlust to finish them off. Mages can instantly kill any Death Knight or Dragon with Polymorph. The Orcs really have just one severely damaging attack, attacking with Bloodlusted Ogres, and Human players playing Orcs always know that is coming. The Orc player in contrast doesn't often have any idea what the Human player is going to do. This Orc player overconfidence and predictability can sometimes be their downfall.

Upgrade Cost
On maps with less Lumber such as Oil is the Key and Nowhere to Run, the Blacksmith upgrades which require less Lumber than the Orc counterparts can be a slight advantage.

Healing
Humans can use Healing to keep expensive units such as Mages and Gryphons alive battle after battle. If units survive an attack, Healing can make them brand new again.

Holy Vision
Humans can use the Paladin spell Holy Vision to spy on the enemy while the enemy can do nothing to stop it.

Exorcism
While the Orcs do not have a very good counter against Mages other than directly attacking them, Humans have several good counters against Death Knights. A Human player often has plenty of Paladins and once the Exorcism spell is researched, there is plenty of mana and Exorcism to go around. A Paladin needs only to get within 10 of a Death Knights to cast the spell. Orc players often stop building Death Knights once Exorcism comes out because of the fear of Exorcism.

Mages Have More Effective Spells Than Death Knights
All Human spells are useful for specific purpose. Mages have more spells that are effective in everyday playing. Death Knights have more "trick" spells such as Whirlwind, Raise the Dead, and Unholy Armor which really don't come into play as often. The only real advantages that Death Knights have over Mages is their one extra attack range and Unholy Armor, Hasted Death Knight "Mage Bomb" trick. While Death Knights can use Death Coil to heal themselves, Human players can just use Paladins to Heal their Mages which is much easier than trying to find an enemy to Death Coil to heal the Death Knight.

Invisibility
Invisibility is an exclusive Human spell that Orcs can do little to counter. One way to fight an Invisible unit is to use attack ground with Catapults and Juggernaughts and hope you hit them but this really isn't an effective counter and is something that is usually pulled off once in a life time. Runes can be thought of as the best counter to invisible Mages.

Humans can use Invisibility with all sorts of spells and units to make nearly uncounterable attacks. One of the most effective Spell combos for the Humans is the Invisible Mage that casts Blizzard on the enemy Peons mining Gold. The Orc player can attempt a Hasted Death Knight Death and Decay attack but this is nowhere as effective as the Invisible Mage attack.

Slow
Humans can instantly cut in half the attack power and speed of any unit. One Mage has enough mana at full to to cast five Slows.

Polymorph
The Orcs have no instant kill spells other than Runes. Humans Mages can instantly kill nearly any unit with Polymorph, especially Death Knights and Dragons. Orc players sometimes get in situations where the enemy has Death Knights or Mages and they have no real way of getting in to kill them other than direct attacks with Bloodlusted Ogres or a complicated spell combo of Unholy Armor and Haste on a Death Knight using Death and Decay. Often the Mages just survive and Heal using Paladins. But Human Mages can usually clear out any group of enemy Mages and Death Knights with Polymorph. Using an Invisible Transport the Human player can land a Mage, pop out, Polymorph, then get back in the Invisible Transport and run. For longer distances, another Mage in the Transport can pop out, make the Mage Invisible, then the Invisible Mage sneaks in, casts Polymorph, then runs back to the Invisible Transport. This is very hard for an Orc player to counter. Walls and Runes are really the only defense against this and not an effective one at that.