said by Jay_: Yep, gray quests give you NO xp. said by Descent:
better to turn in right away, asap. as you go up, the experience points dont go up or down, but the amount required for you to level up does.
The XP DOES goes go down. Hence the color scheme in the quest window. A red quest will give you more exp then a orange quest... which in turn gives you more then Yellow.. etc all the way down to grey.
Always turn in your quests as soon as you finish them. Dont hang on to them.