He was a middling defensive catcher, but Posada did other things, like draw walks and hit for power with his pee-stained hands. He was overshadowed by famous teammates, which is a recipe for Hall of Fame injustice.
Four catchers have had at least ten seasons with 20 or more homers: Johnny Bench and Mike Piazza (eleven each), Yogi Berra, and McCann. Pretty good company. McCann was the only one in that group to never drive in 100 runs. However, his eight 70-RBI seasons were exceeded by only seven catchers in history.