Why did American colonists criticize the Stamp Act of 1765?

The Stamp Act, like the Sugar Act before it, was passed by Parliament to raise revenue. The reason the Stamp Act was more heavily criticized by the colonists is because it was relatively far reaching, it was viewed as raising money for England, and it was passed without any representation for the colonies. Taking these in turn may be helpful.
The Stamp Act was more far-reaching than other tasks. While the Molasses and Sugar Taxes had placed levies on sugar imports, the impact of these taxes was focused. Sugar was not a widely available staple like today, so the tax largely hit wealthier individuals and the rum industry. The Molasses Tax had ample room for getting around the tax, while the Sugar Tax increased enforcement but remained relatively narrow in scope. The Sugar Tax did give rise to some unrest, but it was quenched. The Stamp Act impacted essentially any activity that required printed paper. This impacted colonists in every class and it impacted colonists in every colony. The stamp used as proof of payment also made it difficult to accomplish legal activities without complying.
While all taxes focus somewhat on raising revenues, the prior levies had been viewed as ways to regulate commerce in the colonies. The Sugar Tax was viewed as a way to both regulate the commerce of the colonies and raise money for England, which is why there was some unrest following its passage. However, the Sugar Tax was imposed following the French and Indian Wars after England had spent significant sums of money defending colonists, which may have undermined some of the complaints. The Stamp Act, by contrast, did not appear to regulate any commercial activity, and it was passed at a time when colonists did not think England's need for money was due to the colonies themselves. This made the Stamp Act seem like a means of extracting more money from the colonies.
No colonial representation was afforded when the Stamp Act was passed. This made it look to the colonists as if Parliament believed colonists did not have the same rights as individuals residing in England. The colonists were adamant that they were English and deserved the same rights as the English in England. By passing a revenue raising measure specific to the colonies without allowing for colonial representation, the English were essentially placing colonial residents below the residents of England.