What the heck happened?

I saw this movie on two seperate days. I had to go somewhere, so I couldn't watch the last ~30 minutes. Well, up to that point, I thought I had found a new favorite movie. I watched the end of today, however, and...wow, what the heck happened? A complete tone-shift, no closure to events or characters, and overall...it just felt kind of pointless.

I'm not going to be melodramatic and start saying I hate the movie now. I mean, the first 3/4ths of it was really great. But as much as I want to really like it, overall, I was disappointed.

Am I missing something?