I think there are a few reasons for this:
1) I was so delighted at the time to see the characters I loved appearing on the big screen.
2) Apart from the pace, I think there was a lot to like. The movie looked great. The visual and sound effects were great. The score was great.The development of the Spock character was great.
3) When I was on my way to an interview for my second job (back in 1981) I stopped off in the Brown Thomas shop (then called Switzer's) and they were playing the movie on a LaserDisc player. I was early for my interview so I stayed for a while and watched some of the movie. When I left and headed off the the interview I was feeling great. No wonder I got that job!
One of the people who doesn't seem to have great things to say about that movie is Leonard Nimoy. Here is what he had to say:
I think he [Robert Wise] and Gene Roddenberry were looking for a [2001: A] Space Odyssey kind of thing, like [Stanley] Kubrick had done. A cold, cool “we’re out here in space and it’s kind of quiet and things move very slowly.” [laughs] There was a lot of that and a lot of cerebral stuff. There wasn’t enough drama. It just wasn’t a Star Trek movie really. It had the Star Trek people, but it didn’t use us as Star Trek characters very well.And here is the video where he said this (starts at 5:39)