Since attending my first other ballpark in 1985, I've missed a number of them who have been torn down. Here's a list: [update: I was so tired when I wrote this, I missed a few. There are twelve in total.]
Exhibition Stadium, Toronto; Memorial Stadium, Baltimore; Municipal Stadium, Cleveland; Arlington Stadium, Texas; Fulton County Stadium, Atlanta; Tiger Stadium, Detroit; Astrodome, Houston; Three Rivers Stadium, Pittsburgh, Riverfront Stadium, Cincinnati; Veterans Stadium, Philadelphia; RFK Stadium, Washington D.C.; Shea Stadium, New York. Unless I go to Minneapolis this year, I will add the Metrodome to the list to make thirteen.