The number 52 appears in every episode of the first season.
When Barry is showing S.T.A.R. Labs his power, the camera pans to a billboard for Ferris Air. Ferris Air is the employer of Hal Jordan, the alter ego of Green Lantern.
Mark Hamill has played all on-screen versions of The Trickster/James Jesse including the original "Flash (1990)" TV series and "Justice League Unlimited (2004)," as well as this one. His costume from the original series was also shown in his latest incarnation during the episode "Tricksters".
As a nod to "The Big Bang Theory (2007)," Cisco is seen occasionally wearing a 'Bazinga', a 'That's my spot' or a 'Rock/Paper/Scissors/Lizard/Spock' t-shirt in homage to Flash-loving, t-shirt wearer character, Sheldon Cooper. This creates a plot hole (of sorts) as Sheldon Cooper wears flash t-shirts and talks about the tv show.