Bloodlust
Bloodlust can turn one Ogre into the attack power of three. Bloodlust also triples the damage of Dragons. Bloodlust can allow Death Knights and Troll Axe throwers to take on Dragons by themselves. Bloodlust is almost always freely available once researched as there are often plenty of Ogre-Mages around to cast it and at full mana an Ogre-Mage can cast 5 Bloodlusts. Bloodlust is an offensive spell and is much more effective in increasing the attack power of the Orcs, than Healing, a more defensive spell does for the Humans. Bloodlust can often turn the tide of the game if the Human player is slow to Castle or does not build a lot more Paladins to compensate. Bloodlust is also instantly effective as the Orc player usually already has plenty of Ogres as the Bloodlust spell is researched. Healing in comparison only helps the Paladins out if they survive the battle.

Orcs Are More Intimidating
A group of Bloodlusted Ogres and the Bloodlust sound makes it a much scarier experience then facing a group of Healing Paladins. Although some might argue Human Invisibility and Invisible Blizzard attacks are much scarier.

Runes
Orcs have the unique ability to use their Ogre-Mages to lay down Runes. Runes can be placed in the paths of oncoming enemies to damage them and reduce their numbers before the two sides actually connect. Runes can also be placed in the middle of a Peon Mining stream to quickly take out a number of them. Runes can be placed around Mages to keep them from retreating. Combined with Bloodlust, Ogre mages are very powerful.

Runes can also be cast on strategic locations to prevent invasions and attacks. This is one of the few spells that can actually cause an enemy to retreat at the sight of.

Upgrade Prices
The Blacksmith offensive upgrade prices for the Orcs are less Gold intensive. Gold is needed very early for Peons and Grunts, so it's often easier to have a mix of Gold and Lumber, even tho Lumber takes longer to harvest then a large collection of Gold.

For the last offensive Orc weapon upgrade, the price is 1500 Gold, 300 Lumber. Humans in comparison is 2400 Gold, no Lumber. Since Ogres and Grunts are heavily Gold priced (800, 600 Gold respectively), there is rarely a large collection of Gold built up. While it's faster to mine Gold than Lumber, quite often you have a collection of Lumber built up by this point in the game making the upgrade feel cheaper than paying 2400 Gold.

Hasted Death and Decay
Death Knights can cast Haste on each other to speed up the damage of Death and Decay. A Hasted Death Knight can quickly cast Death and Decay making it easier to attack collections of grouped units before they respond, and also increasing the chance the Death Knight will have, to fully cast the spell before being killed. The Death Knight can also more effectively pull off the "Mage Bomb" by casting a Death and Decay on the enemy then quickly casting Death and Decay on itself severely damaging and killing units that respond to the Death and Decay using Haste.

Hasted Peons
Although largely useless, Hasted Peons can repair buildings faster as well as mine Gold and chop Lumber at a faster rate.

Ready for Battle Much Quicker
As the Orc player gets to the Fortress/Castle Stage, they only need to build one building: Altar of Storms and upgrade their Ogres. A group of Peons repair-build the Altar of Storms to quicken it's construction which gives the Orc player an even bigger jump. Then after two short upgrades: Ogre-Mages, Bloodlust, the power of the Orcs are virtually tripled. At the Castle stage the Human player is more vulnerable to attack as they must alter the makeup of their forces. Resources are a big hindrance here as the Human needs much more before their superior magic can come into play. They need more Peasants and Gold to have as many Knights and Mages as needed to oppose Bloodlusted Ogres. Orc players do not suffer from this problem except when facing other Orc players with Bloodlust.

Resources
Humans sometimes require a lot more to effectively counter Orcs. They either have to build a lot more Paladins to counter the Bloodlusted Ogres, and/or build Mage Towers and a Church, Mages and must research Blizzard which require a lot of resources. Then they need to research Invisibility or somehow sneak Mages in to Blizzard Orc Peons. The Orc player often only needs an Altar of Storms to research Bloodlust.