Just re-watched this movie and I just can’t help being disappointed by it. I realize that’s heresy since I loved the series beyond any limit of what is considered socially acceptable, but there you are.
Having pondered it a bit for a while now, I think there's no single cause, but probably a lot of tiny one that by themselves would be unnoticeable:
* The soundtrack was done by someone different from the TV show, and made use of no themes from the show, excepting a 15 second audio cameo in the very end of the credit roll. The STYLE of the music is different, too.