In the past decade, 80% of baseball’s 30 teams have made it to the playoffs at least once. That leaves six franchises that haven’t appeared in a postseason game in the last ten years: the Mariners, Orioles, Blue Jays, Pirates, Royals and Nationals/Expos.
It’d be a surprise if the Mariners, Orioles, Pirates or Royals made a postseason appearance this year, but the Nationals and Blue Jays could contend, especially now that there’s a second Wild Card berth in each league. That said, the NL East and AL East project as tough divisions, so you could make a case for a number of teams (most recent postseason appearance in parentheses).