And while I'm thinking about it, all the Star Trek movies were underwhelming.
I think maybe there are two kinds of stories (mainly): ones where things happen, and ones where people change. For instance, in any of the Indiana Jones movies, things happen. The hero finds an artifact, fights a villain, kisses a girl and if there's another movie, he'll do it all again. Things happened, but Indiana Jones is basically the same guy, with a few new scratches. In the other kind of movie, people are changed. At the end of L.A. Confidential, the hooker has settled down with her man, the idealistic, young cop has killed in cold blood and isn't sorry for it, and the cynical, thugly cop feels bad about beating people up. They're new people, permanently changed by their experiences.
The problem with a series going to a movie format is the why of it. If it's a things-happening sort of movie, why not just have another episode or two of the series? That's why the Star Trek movies suck; they're just long episodes of the series. But if a series, with beloved characters, has a movie made of it and the characters are permanently changed, then it pretty much ruins the series.