"The Motionless Picture" wasn't an uncommon nickname, certainly... I think it did well in the theaters only because it had visual effects of the sort that even Star Wars didn't do, but more on that later...
The story is pretty much a done-better version of mashed-up greats such as "The Changeling" and "The Doomsday Machine".
But perhaps, are we all so accustomed to movies having to be whiz-bang-wham-bam-thankyarabid speed? Well, perhaps... but at the same time, a lot of us who first saw the movie as being a snoozefest would try it out years or decades later and something goes *ding* and now it's high on our lists and can actually sit through it. Not too often since the movie's biggest strength is in the visual elements, which don't hold up as much as intricate plotting or character quotes...
I'm also one of the few out there who prefers the extended TV cut as it added in tons of material, some of which is actually relevant to the plot... or at least renders some scenes less choppy in feel. After all, this is the same flick where its final scene shows McCoy and Spock having uniform colors that get swapped for a cutaway and then put back in the next scene...
Back to why I think it succeeded despite all its problems -- being a product of its time and thinking along that mindset helps. It's 1979, we'd all seen the same 79 episodes as reruns. All of a sudden, there's a shiny new film with the shiny old crew. A big film! A big crew! With a new standard of visual effects than the original could never achieve! Slow-moving spaceshippron really hits the spot.
I'll agree that the shuttlepod trip to the ship IS probably the least-effective scene on repeated viewings, but as an initial viewing experience it was an incredible mindblowing spectacle. With just enough of a plot to make the film feel more of an event and not gratuitous, indulgent shlock.
