Not sure how much work it would be or even if the time/benefit is there for SSG but I would like to see the Progress column on the Adventure Compendium expanded to include little gems or orbs that light up as you complete the different difficulties.

Instead of just showing the highest difficulty completed, you would see 4 orbs that start out blank but then light up as you complete the adventure on that difficulty for the first time. Blue (Solo/Casual), Green (Normal), Yellow (Hard), Red (Elite). You could even have a little Skull Icon to indicate Reaper difficulty. If you wanted to get fancy, you could then have different Skulls based on the Reaper Level completed.
--For those with both Heroic and Elite/Legendary difficulties, I would like to see two sets of orbs or maybe the orbs could be bisected so we could track both.

The game knows if you have done a quest on a certain difficulty for the first time as seen on the XP sheet, so the data is there.

This would give players an easy indicator to grab first time bonus on quests without having to jump into the quest itself. It would also give those who like deeding (Lotro term) a little something to work towards, i.e. having all lights turned on. I know, not much of a challenge to go back and run every quest on Casual but a time sink is a time sink.

If you are feeling generous, you could add a reward for having everything lit up. Some cosmetic like a pair of glasses or book off hand to show you are a student of the game.

Just an idea as I run the TR train.