In 80 years, what changed between the original series and TNG?

Yeah, between the original series and Star Trek: the Next Generation and Deep Space Nine and Voyager, it was about an eighty year time frame in our fictional Star Trek universe. One of the reasons that Gene Roddenberry decided to put those shows into the future of the original series is he knew that a lot had happened in the real world, in the 20 years between the original series and the Next Generation, and he wanted to incorporate some of those new developments in science into the new shows.

Take, for example, the holodeck. Virtual reality technology did not exist in the 1960s, so we never really saw or heard anything about that in the original series except in a kind of an oblique and accidental way. But holography and the idea of virtual reality was very much established by the time the Next Generation came on the air.

So Roddenberry thought; 'Well, let�s � put this thing called a holodeck onto the ship. It would make sense that crews would need recreation. They would want to at least have the illusion of being on a planet during these long stretches of travelling through the vacuum of space.'

And while he was at it he decide our warp drive is probably going to be more efficient. It�s going to go faster and we'll be able to travel greater distances more quickly. Transporters will have more range, so they�ll be able to do site to site transport very easily � to beam someone from the bridge directly to engineering or from engineering to the surface of a planet.

This was an example of the sort of radical idea that only showed up in one episode of the original series, and was considered very dangerous, that was a common application for this transporter in Next Generation.