Firstly, when you complete a quest you want to know what you got and you might not notice. I also think that if you complete a quest with full inventory you won't get anything, but I'm not sure about that.
Secondly, after you do what you are required to do, you want to find the NPC wo gave you the quest. I've been struggling with it because I'm going to very far places and going back can be tough.
Therefore, I think the quest menu should show the coords of where you got the quest, and what you got for it if you have completed it. You can also show date of quest for those who like to complete them by date or if they will have expiry dates.
Thank you for your time, Tom Markovich.