Simple Idea:
You JUST leveled to 14, but you REALLY want to be 15 and run Ruins of Gianthold with your friends. So you go to the trainer of your choice and click level up as a (class.)
There is a prompt:
You do not have enough experience to level normally, and are 97% from level 15. Do you want to level up and take an XP penalty of 50% (after all other earned experience modifications are taken into account) until you are 3% from level 16? (YES/NO)
You click yes and DING! Level 15. An icon appears in your debuff bar with the total amount of experience that is needed to be earned before the xp penalty vanishes. While you have this debuff you cannot deficit level again.
The biggest drawback to this deficit level is that if you're 10k xp from ending the penalty and you earn 125k xp, even though the first 10k might finish the deficit level off, the entire 125k would be penalized 50% so there's a little bit of planning necessary to maximize the benefit and minimize the pain caused.
(addendum)
The above was a terrible example of why this would be used. Instead, imagine a 19 wanting to do epic quests with his level 20 friends. or you just don't want to be level 8 anymore because you hate the quests at that level, or there's some gear you REALLY want next level and you don't want to wait...
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Like I said, simple.
It also is more experience to do it this way instead of just taking the xp you would have needed and lumping it into the bar, because the previous levels' xp is always easier to satisfy at higher levels and it would be much less a penalty.
Anyway that's it. Hate away...