I discovered Star Trek when I was a kid around the same time I discovered Doctor Who, and I've been a sci-fi nut ever since, and if there is one thing you can count on from a sci-fi nut, it's an opinion on everything sci-fi.
Here's my two cents.
The main problem with Star Trek: The Motion Picture, and don't get me wrong I love it, is this...
Gene Roddenberry was in the process of bringing Star Trek back to TV. They had sets designed, and were working on a new ship model and were working to resurrect the show.
Leonard Nimoy decided he wanted out, so they created a new Vulcan character to take his place, and so on. Then Star Wars became such a big money maker, that the suits at Paramount, who up to that point had little use for Sci-fi asked the question, "What do we have that could compete with this?"
The new series was scraped and a feature film was put into production. Once that happened Leonard Nimoy wanted back in, and the Vulcan character that was going to be in the new series was killed in a transporter accident at the beginning of the film.
One of the biggest problems with the film as I remember it, was that the studio wouldn't budge on the release date and the film went out before really being completed.
I remember watching it as a kid, (and I watched it and re-watched it as I do all my sci-fi shows. I'm re-watching Star Trek Voyager right now actually.

) and I couldn't help but laugh at the scene where... I think it's when Kirk was going out in the space suit to go after Spock, and you could see the scaffolding and soundstage because the CGI hadn't been finished.
Editing is a very big part of making a movie. The pace of the movie is set in editing (unless I'm just talking out of my ass here) and they didn't really have enough time to spend on that with this film. You add to that a creator with one vision, a Director with another, and a bunch of studio suits who just see dollar signs, and...
Again I loved it, and if I can go all super geek here...

I've always been one of those people who thinks... at the end of the movie when Decker and the v-ger version of Ilia get it on, and explode in an orgasmic joining of man and machine, that they very well could have found themselves on the other side of the universe and with that joining of man and machine, been the birth of the Borg.
Well, my time and two cents are up.