This is the first year that both teams in the finals are expansion teams: New York Mets v. Kansas City Royals.
Modern expansion started in 1961. Fourteen teams were created through 1998 bringing the total to 30. Each of the current five team divisions has at least one. Four in American West, three in National West, East. Two in American East, one in each Central.
1961: AL: Washington Senators (became Texas Rangers in 1972), Los Angeles Angels (became the Los Angeles Angels of Anaheim or something)
1962: NL: New York Mets, Houston Colt45s (changed name to Astros in 1965)
AL: Kansas City Royals, Milwaukee Brewers (first year as Seattle Pilots in 1969)
NL: Montreal Expos (became Washington Nationals in 2005), San Diego Padres
1977: AL: Toronto Blue Jays, Seattle Mariners
1993: NL: Florida Marlins, Colorado Rockies
1998: AL: Tampa Bay Devil Rays, NL: Arizona Diamondbacks
Expansion teams to reach the old World Series/finals and the years it took:
1969 Mets (won) 8
1980 Royals (1969) 12
1982 Milwaukee Brewers (first year as Seattle Pilots in 1969) 14
1984 San Diego Padres (1969) 16
1985 Royals won
1986 Mets won
1992 Blue Jays won
1993 Blue Jays won
1997 Marlins won 5
2001 Diamondbacks won 4
2002 Angels won 42
2003 Marlins won
2005 Astros 44
2007 Rockies 15
2008 Rays 16
2015 Royals v. Mets
The expansion teams don't do as well as the 16 original teams but they've done better than might be expected given that most play in smaller markets. In addition to New York, Houston and Toronto might be considered big market.
Quickest to the finals: Diamondbacks, Marlins, Mets
Most often: Mets 5, Royals 4
2 championships: Mets, Blue Jays (consecutive)
Finals 2 consecutive years: Blue Jays, Rangers
Padres: only other in finals twice, losing both.
Never in finals: Mariners (0 for 39)
The Mets won their first pennant (and championship) in the first year the Royals existed: 1969. So the Mets have five pennants in 54 years, the Royals four in 47. Mets every 10.8 years, Royals every 11.6. Pretty close.
Mets and Royals are the most successful expansion teams since 1961. May the best expansion team win in 2015.