I like fun, character growth, and a general good triumphs over evil message. Internal consistency is helpful but not necessary. (The magic system in Robert Asprin's Myth series is all over the map but the books are fun.)

I also have to like the characters. If I don't like the characters, it's really hard for me to stick with a story. That doesn't mean the characters have to be "righteous" but I should want to spend time with them. In the series I keep going back to (like LOTR), it's because I like being with the characters.

I really like the Honor Harrington series from David Weber, in part, because the pseudo science of space travel is pretty interesting and I like Honor as a character.