TMP isn't enough like the series or the following films to be a "fitting" first Trek movie. I still like it, but it's more like an odd AU than a piece of the puzzle.
I agree. People went to see the first Star Trek movie because they liked TOS, and then what we got was ... nothing like TOS. Kirk isn't at the top of his game, Spock is off trying to starve his emotions into submission, and the characters spend WAY too much time staring at the viewscreen in awe and way too little time interacting with one another or trying to solve the problem. It felt to me as if the special effects were the star of that movie, and the human stars were extras in their own film.
And while "The Changeling" was a reasonably good TOS episode, it wasn't top-ten material. Why make an entire movie based around the same story? Plus, I thought "The Changeling" was actually a more interesting version of the story.
I think it's fascinating the way the creators of some of the most beloved science fiction series seem NOT to understand what they themselves have made or why people liked it. Roddenberry gave us a Star Trek movie that many fans disliked, and George Lucas gave us a second Star Wars trilogy that's almost universally regarded as much weaker than the first one. How could they create greatness and then not quite understand what they, themselves have made? Just one more way in which people are really interesting.