Not that I’m complaining, but I don’t remember getting full reputation for completing grey quests prior to WotLK.
Maybe it’s just because I was only 6 levels higher than the quests in question, but I remember getting only 4/5 of the reputation for that back in the day. (And 3/5 rep for being 7 levels higher, etc.)
Upon further questing, I have found that by completing “Grove of the Ancients” (a level 11 quest) at level 17, I do indeed gain 80% of the usual reputation reward. I don’t know if this is because of a recent update to the beta or some oddity with the other quests I had completed, but there is indeed an incentive to complete quests before they go grey, as there was before.
I can confirm that rep decay was introduced in the WSG update to the Beta on Friday. Earlier that morning I ran some tests on known quests and they gave full rep when gray. However, after the WSG patch later that day, I ran some more tests and verified that rep decay is now in effect for gray quests. You can see the results of my tests in this thread:
I suppose I should add that it’s not necessarily the color of the quests but the level difference that counts. Rep decay kicks in when you are 6 or more levels higher than your quest is. At low levels (6 and 7), quests turn grey when you are only 5 levels higher than the quest, but you can still do a level 2 quest at level 7 and get the full experience and rep for it.
I assume that by the same token, at higher levels when it takes longer for quests to turn grey, you’ll still get decayed rep for completing a quest 6 levels below yours even if the quest is green to you.