You have to understand, it wasn't the movie itself, but what it represented: a sci-fi franchise that was literally raised from the dead because of fan interest.
As someone who saw the film in theaters, I was elated to see these characters again after years of watching re-runs. And what they were wearing, how it looked, the whole premise was very futuristic (for the 1970's).
Yes, it's incredibly dated, and yes, there was a bit too much reliance on the special effects (and the whole thing with the transporter accident was just to appease NASA), but it was still, at its heart, a damn good story.