The quest journal UI shows quests in the order they were added to the player's quest journal.
The only other thing that affects the order in which quests appear are groups. Quests in groups are listed first. Quests without groups are listed at the bottom.
Would it be useful to provide some sorting options? If so, what sorting options would be useful? Just alphabetical?
Should there be an option to sort all completed quests to the top or bottom? Currently they're mixed in with active quests.