Different movie, different time. There was a time when not everything on screen had to have an action sequence ever 15 seconds to make it "watchable". There were a lot of movies that came out in the 60's and 70's that had similar pacing. We look back at them now and call them bad movies because they're "slow". The audience used to have a much longer attention span, I think.
The world, in general, has turned into a very immediate, "Right NOW!" culture. I know people who can't manage to wait for literally 10 seconds for a programs to open on their computers. You couldn't make a movie like TMP today with that sort of pacing and of course in retrospect, it looks slow. I would imagine that when it came out, it was pretty on par with what was typical of the time, for that genre. Remember that Star Wars, just a few years earlier, broke all sorts of rules for what was considered "acceptable science fiction".
By the time that Wrath of Kahn came out (and there are still slow periods in that movie) the world was no longer accepting of movies that showed long scenes of visuals or scenes with little movement to them.
Now, in 2009, we have movies that are paced so fast that you can't take it all in in a single viewing. They're meant to be re-watched. Movies 30 years ago couldn't be re-watched other than at the movie theater or eventually on the Sunday-night movie. We didn't have the luxury of going home and watching it again. You couldn't rent it.
If a movie is actually paced so that it keeps the attention span of this ADD, Commercial length, super-fast cell phone culture, it's blasted as being "too heavy on action" but if the director takes the time to really create a beautiful scene, stretches things out, or doesn't pan the camera around 60 times a minute, the film is blasted as being too slow and unwatchable...Hollywood can't win.
I liked The Motion Picture (Star Trek I). It don't think it was that slow and had an interesting storyline, especially with Decker and the Bald Girl.
No, there wasn't any mean and crazy villian, no great shoot outs or classic lines. There was no CGI to rip out your throat. I liked it, and if you didn't, well, then yhou can kiss my green behind.
I really must object to this "either you like TMP or you have a short attention span/ADD and just want to see explosions" attitude.
I very much enjoy the vsual spectacle of 2001 - one of the archetypical "slow" movies. I enjoyed watching Last Year At Marrienbad (sp?) once (not sure I'd like to see it again).
I also enjoy TMP - however I can see that it is not what the average film goer would expect from a Star Trek movie, if they had only seen TOS.
The film attempts to have something to say, however it hides that behind endless reaction shots and pretty pictures, needless inane dialog (just how many times is Kirk going to talk about the viewer?) and then ultimately tries to emulate the "star child" ending of 2001 with the Decker/Illia light show.
I agree with earlier posters who suggested the Robert Collins ending could have played better.
In a way, it is like The Cage - heavy Reddenberry involvement and the following Trek productions went in a different direction.