Final answer:
Oliver Cromwell's ability to work with Parliament and promote religious toleration was key compared to Charles I, who failed to reassure the English people and led the country into division and weakness. Cromwell understood the need for balance between monarchy and Parliament, which Charles disregarded, leading to fiscal chaos and internal turmoil.
Step-by-step explanation:
The most important quality that Oliver Cromwell had that Charles I lacked was the ability to work effectively with Parliament and to garner support for religious toleration among varying sects. Charles I's inability to adopt policies that reassured the English people of their traditional religion and civil liberties, as well as his lack of sound and responsible leadership, led to England's decline as a European leader and severe internal divisions. In contrast, Cromwell, though he became a military dictator over time, initially had the support of various factions, including the army, the nobility, and Puritans, and understood the balance required between monarchy and the legislative body.
Oliver Cromwell's stance on religious toleration further highlighted Charles's failures, particularly his marriage to a Catholic princess and the changes to religious practices which alienated many, especially Puritans. Even after Cromwell's death, the lesson learned was clear: the necessity of cooperation between the Crown and Parliament, a balance that Charles I had severely disrupted during his rule. This recognition of a need for constitutional balance and concern for the rights of subjects was a significant attribute contributing to Cromwell's initial support compared to Charles's decline in popularity.