What is the difference between gear drive and the idler system?

Gear driven means that the reel tables (posts that take up tape) are driven by a gear which meshes with other gears, as opposed to an idler (rubber disk) that pushes up against a brass bushing and the back of the reel tables. The earlier transport, my favorite by far, used a large idler. With the BX, CR, MR series decks, as well as the RX-202, Nakamichi tried to save money by building a much lighter weight transport. The idler in these was a mistake. Nak later provided a gear upgrade kit that could be installed. It wasn't that it was terrible but the idler needed replacement every 2-3 years whereas the earlier models only now, 15 years later, are showing signs of needing new idlers. The gear update is quite nice and is now standard on later model CR-decks, Cassette Deck, and DR-series.