The Mongol invasions were fundamentally different in their intentions regarding the conquest of enemy lands. Wherein Roman, Persian, or Arabian empires conquered land and taxed it's people (seeking material growth), Mongols had little intention to preserve foreign landmarks and environment for their use.

During their conquests Mongols sometimes killed off the ruling elite, placing their own on the thrones of their newly acquired vassals. Many historians would say that they lacked the "culture of chivalry", giving no interest in gallant behaviour towards their captives. Mongols usually executed their captives and let few enemies flee to tell others about the Mongol terror which resulted in a lowered state of morale. Sometimes this tactic resulted in these stories about Mongol destruction to take on and grow a little. Mongol conquests were destructive but slightly exaggerated.

In a way the Mongol invasions were comparable to German blitzkrieg; Their armies were considerably faster than their enemies due to their cavalry tactics. This resulted in a huge advantage in medieval combat, making Mongol armies overwhelming to their enemies.

Source: Maailmanhistorian pikkujättiläinen (A finnish academic book made by several history professors)

When the Mongols attacked a city or area they often gave the inhabitants the option to surrender. Often when they did only the royalty were killed. But what gave the Mongols the reputation was what happened if you did not surrender. According to their own history "The Secret History of the Mongols" they would slaughter or enslave the entire population. Even sending men back days later to find anyone who had been hiding.

Sources other than the Mongols confirm this. John of Plano Carpini, one of the first Europeans to make it to a great Khan's court writes about finding piles of bones so large they saw it from a distance and thought it a mountain. He said the ground was so saturated with the decaying remains that they had to travel some great distance around because their wagons would sink into the ground.

Muslim writer Juvaini describes how citizens were divided up equally among the Mongol soldiers to be slaughtered, literally like animals.

The difference between Mongols and other empires was that, while the Romans, for instance, might deal out this sort of treatment to a hated enemy like Carthage, it was rare. The Mongols would do this to any city that stood against them.

There is a story of a Mongol general, I believe it was Subutai, who, when passing through a region demanded a city supply him. This they did. When he came back through he demanded more supplies, which they claimed they did not have, so he ordered them all killed.