Edit
TheRodai already proposed this in a prettier fashion about a year before I started playing DDO in earnest. All the more reason to support it!
What I Propose
Show us the number of times this character has completed a quest on each difficulty. For example, "3 Normal - 1 Hard - 1 Elite."
-Display these numbers above or below the corresponding difficulty selection boxes.
-Display these on the screen where we can search for the quests by level. For example, instead of just saying "Elite" is complete, show "3 Normal - 1 Hard - 1 Elite."
Who This Affects
-This change affects new characters created after this change is implemented.
-The highest difficulty completed can still show on the quest search screen; however, quests completed before this change will be in a separate category of "Unrecorded" or "Unknown."
Why?
Often, I've done certain quests with various groups, not remembering how frequently. I'd be disappointed to see a very small XP gain or that chests were empty because I'd done it so often. Also, this helps in leveling so I know the best way to get XP; that is, 3 of 1 difficulty, 1 of another difficulty, and 1 of the last. For example, 3 Normal - 1 Hard - 1 Elite.