Ellis first lists Adams's obvious qualifications for the presidency. Adams had "made American independence his life's project," and he was in many ways made by the revolution. As Ellis points out, he, like others in his generation, most notably Hamilton, would have "languished in obscurity" had he been born in the Old World. Adams's contributions to the new nation were virtually unmatched. He advocated earlier than most for independence, he served in the Second Continental Congress, and he wrote Thoughts on Government, which informed the framing of many state constitutions, and Defence of the Constitution of the United States, which celebrated the ideas behind the new government. He also represented the nation in France, advocated for Washington to take command of the Continental Congress, and for Jefferson to be the author of the Declaration of Independence. He also, of course, served as the nation's first vice president.
On the other hand, Adams also had many disadvantages. His performance in the office of vice-president, Ellis argues, was actually a negative for him. He lost a great deal of respect from members of the Senate, who eventually made it clear that they had no intention of allowing him to actually participate in floor debates (the Vice President also serves as presiding officer of the Senate.) While he "steadfastly supported" most of Washington's policies, he found himself marginalized in the major debates between Hamilton and Jefferson. Adams had also made several ill-considered, and public, remarks about monarchy, which opened him to charges of monarchism from his Jeffersonian critics during the election. In short, Adams was a man who tended to speak his mind, and he was largely lacking in the sort of tact that mattered in forming political relationships in the early republic.
Source: Joseph J. Ellis, Founding Brothers: The Revolutionary Generation (New York: Vintage Books, 2000) 162-177.