The most repeated grievance against King George III by the American colonists was his repeated refusal to recognize them as true Englishmen. He did not allow the colonists to govern themselves as equals, rather than mere colonists.
The Declaration of Independence, approved by the Continental Congress in 1776, is a concise list of the grievances that the colonists held against King George. The document states repeatedly that he disrupted their rights to due process not allowing judges to make independent decisions, refusing to allow his governors to pass laws and refusing to enforce laws that were already in place that benefited the colonies.
They had brought these grievances before King George long before the Declaration of Independence, having sent him a letter in 1774 that listed the above issues and asked for the king's help in solving them. Like the Declaration, this petition highlighted the fact that the colonists were being treated as subservient to the British empire. The petition was delivered by Benjamin Franklin directly to Parliament and King George, and it was summarily rejected. This rejection lead directly to King George declaring his American subjects to be in "open and avowed rebellion" against the crown and contributed further to the grievances that the colonists had against him.