On Earth-2, when Barry calls his mother back, on his phone below contacts for Mom and Dad are contacts for Bruce, Hal and Diana (a nod to Justice League members Bruce Wayne (Batman), Hal Jordan (Green Lantern) and Diana Prince (Wonder Woman)).
When Barry, Cisco and Harry travel to Earth-2, Supergirl makes a cameo through the dimension stream as a reference to an upcoming crossover with Supergirl (2015). Also seen in the breach tunnel are Green Arrow, John Wesley Shipp as Flash from the 90's version The Flash (1990) (both before Supergirl), a Gorilla that is likely either Grodd or Solovar, a gun wielding Jonah Hex, a hand with a Legionnaire/Legion of Superheroes' Flight Ring, and multiple if not infinite Earths.
On Earth-2, Barry is surprised to learn that Joe's doppelganger is an accomplished singer. Jesse L. Martin has appeared in numerous musicals on stage, perhaps most notably in the original Broadway production of "Rent", a role he reprised in the film adaptation Rent (2005).
During a broadcast on Earth-2, the mayor of Central City is referred to as Mayor Snart and as a "he." Leonard Snart is the notorious villain (then anti-hero) Captain Cold on Earth-1.
There are a number of subtle differences between Earth-1 and Earth-2, completely unrelated to the story, that were thrown in to make the world more visually distinct, with the most obvious ones being the art-deco style of the buildings and vehicles, and the 1940's style telephones and clothes. Some other differences are that Earth-2's dollar bills are red and square-shaped, as opposed to green and rectangular-shaped on Earth-1, and HD TVs being oriented vertically instead of horizontally.