For some reason, Big Thunder Mountain feels infinitely more "intense" when it's dark out. I don't know if it's because the ride's been running all day and the coaster/track is "heated up" and moves a little quicker or if it's just psychology playing tricks on my mind, but there's a huge difference to me riding Big Thunder at night.
Seven Dwarfs Mine Train was a bit more enjoyable during the daytime, however, that could be because I've ridden it so few times that I wanted to take in all the details which was obviously easier when it was light out. (However, you can see Snow White in the final scene MUCH better at night without the glare of the afternoon sun which twice now for me has made that scene nearly impossible to see.)
Similarly, I like Splash Mountain a little better during the daytime as I find the outdoor scenes a bit more enjoyable then. Plus, I like going up the big lift hill and seeing the "light at the end of the tunnel."
For the most part, though, I love MK at night. Getting on Pirates and walking out into the nighttime adds to the mood despite the fact that none of the ride takes place outside.