Before there was "Jack" there was "Jim".
The "Jack" format is a toned down version of the "Jim" format launched in Las Vegas in the late 1990's. "Jim" celebrated the masterful songsmithing of Jim Steinman, an incredibly talented songwriter and producer dedicated to the idea that if you don't go over the top you can never see what's on the other side. Meatloaf's "Bat out of Hell" was Steinman's first triumph, and since then he's used his unique production style on work from artists ranging from Barry Manilow to Pandora's Box.
Steinman's style celebrates excess. In Jim's world, if a song sounds good with two backup singers and a session band it will sound even better with the Mormon Tabernacle Choir backed up by the London Philharmonic. He also has a tendency to remake his own songs with multiple artists, reinterpreting the music each time to better fit the vocal range of the talent. An example would be "Original Sin", a song he first recorded with Pandora's Box, then re-recorded with Taylor Dayne for use in the soundtrack of "The Shadow", then re-recorded once again with Meatloaf, and then re-recorded with Michael Crawford and Bon-Jovi when it became part of the musical "Dance of the Vampires".
"Jack" takes the over-riding thematic unity of "Jim" and turns it on it's head, celebrating diversity and mediocrity instead of unity and excellence. It's the ultimate "Oh yeah, I remember that song" format, designed to jog the addled memories of an aging audience with nostalgic snippets of their glory days. In fact, I'd suggest that "Jack" is the soundtrack to the world of Springsteen's "Glory Days".
Your friend forever,
Biggus Dickus