I think the XP penalty for repeating a quest multiple times should be changed.
My preference would be to put it on a timer just like the chest ransack timer except with a longer duration. The ransack timer lasts a week. I would say a month would be a good time for the repetition timer.
This would be most helpful for people who get stuck in mid levels (8-12) and are finding it harder and harder to find XP.
Why a month? Because that is long enough to encourage people to go do other quests rather than just wait a week and run Stormcleave 4 more times but it is short enough to minimize the frustration felt by people who get stuck at a level.
This should not reset the bonuses for 1st time completion on Normal/Hard/Elite; just reset the penalty counter so that after a month the first time through you get no XP penalty, than 10% the next time, 20% the next, etc.
If this change doesn't seem like a good idea, I think a different approach would be to either (1) lower the penalty from 10% per repetition to 5% per repetition or (2) cap the penalty at 50%.
All in all I think this would help out players who are getting frustrated leveling up (esp. soloers and static groups that play once or twice a week) without going outside the spirit of the game.
If you haven't done the quest in a month, you can't remember it all that clearly, so there shouldn't be as much of a penalty (I could even see dropping the penalty back to 10% after a month instead of 0). Also, if it makes sense that the loot rewards get reset, I don't see why the XP rewards shouldn't as well.
I'd be curious to hear what people think.
P.S. I'd also like to see the Slayer quests reset in explorer areas, but that's probably another topic...