Leading the way on offense was Rookie of the Year and American League MVP Fred Lynn with his 7.4 Wins Above Replacement (WAR from Baseball-Reference). Lynn is one of only two players in MLB history to win both the Rookie of the Year award and MVP award, along with Ichiro Suzuki. In Lynn's case, there were other players with similar WAR figures, such as Twins great Rod Carew. But it's hard to fault the voters for selecting Lynn who was on the AL pennant winning team. (In Ichiro's case, he wasn't even the best player on his own team; that honor belonged to second baseman Bret Boone.)
The Sox offense did include future Hall of Famers Carl Yastrzemski and Carlton Fisk, but Yaz did not enjoy one of his better seasons and Fisk only played in 79 games. However, Fisk was excellent when he did play, posting a batting line 50% above league average. Left fielder Jim Rice turned in an excellent rookie season (3 WAR) on his way to a Hall of Fame career. The always solid right fielder Dwight Evans was also excellent, and the primary designated hitter Cecil Cooper provided an .899 OPS. Also, Bernie Carbo, famous for his three-run homer that set up the extra innings in which Fisk would hit his historic shot over the Green Monster, had a fine regular season as well. The former Red and Cardinal had a .892 OPS in 107 games, splitting his time between Designated Hitter (DH) and the outfield.
The pitching staff wasn't exactly composed of world beaters, though. Sox legend Luis Tiant didn't have close to his best season. Bill Lee, Tiant, and Rick Wise were only slightly above average starters, but the bullpen was decent. Swingman Roger Maret was also above average. They kept their team in games. With good defense and timely offense, no one had to be extremely excellent on the mound for the team to succeed.
It wouldn't be a stretch to say that Lynn and Rice put this team over the top after a mediocre 1974 season. They replaced the lousy batting lines of Tommy Harper and Juan Beniquez in left field and center field, respectively. This team came nearly as close to winning the World Series as the 1986 team would. But, considering the heartbreak of Bill Buckner's error and Calvin Schiraldi's implosions, the 1975 team is remembered much more fondly.