If I'm reading the heavily edited OP correctly, the proposed system would grant a toon almost the same amount of xp each time he/she completes it. If that is the case, a (IMO) hefty negative result would be a drastic reduction in the variety of quests being run. It's already very common to see "daily" runs of von3, wiz king, and a couple of others. If the xp yield were pretty much the same aside from any ransack penalties, I believe this trend would get a lot worse.
The point of first time bonus is to incentivize variety. You get great xp the first time you complete. Some quests are still worth re-doing to some degree, but there are also other quests that will give a good xp reward on the first time.
Also, pretty clearly this is aimed at people epic reincarnating. I'm personally okay with that taking a while.