It's already implemented - quest items are currently destroyed when you abandon the quest. I tested it yesterday.
There's another post from Tseric/Blizzard relating to XP turnins can be found at
http://forums.worldofwarcraft.com/th...09422187&sid=1
The text, for those of you with restricted access:
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XP Gain from level 60 going into the expansion is still something that is being adjusted. The devs are well aware of many methods and routes by which players are 'stockpiling' XP and don't want to see arbitrary bursts of experience simply because someone wanted to get to 61 first. The leveling curve is being minded carefully for the expansion and already, we've made some bug fixes to that end (players no longer gain rested XP at the level cap).
It is quite possible that some of the methods mentioned and others may be eliminated, devalued or possibly left as is. The particular decisions have not been made as of yet, but they have been discussed.
Just a bit of a heads up for folks who might be hoarding AQ turnins. We're watching. ;)
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I've decided not to spend extra time grinding repeatable quest stuff. I will have 20 completed -regular- quests in my log, though, as I don't believe they will remove XP from those. Anything repeatable that I already have isn't going to be turned in, until expansion, just in case.