Adams was irreplaceable during the Revolution. As a President, however, he stunk. He was our first liberal President, giving us travesties like the Alien and Sedition Acts. But worse, after his loss to Jefferson he packed the federal courts with more big gov't types including Marbury of Marbury v. Madison fame, which, IMHO, led to the downfall of our Republic.
Marbury didn't get the office he was intended to receive, thanks to John Marshall's decision in Marbury v. Madison. Picking John Marshall to be Chief Justice may have been Adams' most important legacy as President.
The election of 1800 may have been the only time that the 3/5th rule (counting three-fifths of the slaves when calculating the number of seats in the House of Representatives a state got, which affected the number of electoral votes) made the difference in the outcome--without the extra electoral votes in the South because of that rule, Adams might have had more votes than Jefferson and gotten another term. Whether he would have agreed to the Louisiana Purchase, I don't know.