From The Flash to The Flash.
Fans familiar with the short-lived '90s TV version of the Flash immediately recognized the actor playing Barry Allen's jailed father in this version. John Wesley Shipp eventually took on the role of Jay Garrick, The Flash of the show's Earth Three. Shipp's fellow '90s Flash veterans Mark Hamill and Amanda Pays have also joined the Arrowverse, as their old characters The Trickster and Tina McGee, respectively.
Happy Birthday, Barry Allen!
Grant Gustin and John Wesley Shipp share more in common than their roles as the Scarlet Speedster. Both men were born in Norfolk, Virginia and both of them have January birthdays. Gustin was born in 1990 the year Shipp filmed his Flash pilot.
Throughout the fictitious town's history in the comics, Central City has been situated in Pennsylvania, Ohio, Florida, and Missouri. The Central City of the Arrowverse appears to be in Oregon. The show has used footage shot in Portland and in its second episode we see a map of Central City that's clearly based on Portland. (No word on whether or not any of the characters from Portlandia exist on that Earth...)
The Flash made room for Green Lantern.
Like its sister series, Arrow, The Flash has given us plenty of indications that Hal Jordan exists in the Arrowverse. We've seen Coast City, home to the Silver Age Green Lantern. There are numerous references in The Flash to Ferris Air, where Hal Jordan works. In the Season One episode "Rogue Air," Barry makes a remark about a test pilot who mysteriously disappeared; clear foreshadowing of Green Lantern's origin.
Harrison Wells is not from the comics.
The Flash and much of his supporting cast, locations, and situations are drawn from the rich mythology of the comic books, ever since the first version of the character appeared in 1940's Flash Comics #1. But despite the prominent role of Harrison Wells (and all of his multiverse variations) in the TV series, this is one character that was created specifically for the show. He's not alone. Eddie Thrawn, Tess Morgan, and Arrow's John Diggle were all created for the TV Arrowverse as well.