When quests are completed you can either delete them or leave them to clog up your quests page. It would be great if there was an option to archive them.
This is because although I would like to see clearly what quests I still have left to do, I still might want to re read the ones I have done, so I can't delete them
Actually, you can see all your past quests. When you complete them, they stay on your current quest list until the end of the day. After that, they go to your past quests list. You can get there on the quests page. Right next to the Add button is an option to Show either current or past quests. Click the scrolly list and switch over, and you can see the archive of the ones you've done!