As most of you probably already know, the World Series pits the winner of the National League (NL) championship game against the winner of the American League (AL) championship game. The first team to win four games in this series is crowned the year's MLB champion. The American League and National League have one major rule difference - the designated hitter (DH). Only the AL permits this tenth player, whose sole job is to bat in the batting order.
The general rule for the DH in the World Series is "when in Rome, do as the Romans do". The DH is only used when games are played in AL parks.
The designated hitter rule was implemented in the AL in 1973. Many baseball purists have disdained it as the ruin of the game. The practical impact of the DH is pretty straightforward. First, it improves each team's offense because the DH is generally a very good hitter and he bats in place of the pitcher, most of whom are terrible hitters. Second, it removes a major strategic element of the game. In the National League, managers are frequently faced with the need to decide whether it is more important to pinch hit a better batter for the pitcher when the pitcher's at bat comes at a crucial point in the game. If the pitcher is doing well or if there are few good pitchers available, this essentially becomes a trade of offensive production at the expense of defense.
The player's union likes the DH because it gives aging sluggers an opportunity to continue to collect large paychecks. Not all designated hitters are aging. Some are just poor defenders. If first base is the last refuge of poor defenders in the NL, Designated Hitter is in the AL.
There is definitely an impact on the World Series because generally only one of the two teams is carrying a person on their roster that is extremely well suited to the position. Since the American League won the All Star game this year, they will host the first two games and the last two games, assuming all seven are played. This creates a slight advantage for them.
The fact that AL pitchers go all season without batting would seem to be an advantage for the NL team in their home games, but this doesn't seem to make much of a difference because most pitchers bat poorly, even if they have more at bats during the regular season.
Watch this year to see how the DH affects the outcome of the World Series.