I think the spirit of the D&D adventure is that a "level up" event should feel like a big accomplishment, no matter what number the new level is. It should not be something that happens three times in the same dungeon. I can remember the original Baldur's Gate when I finally got my mage to level 2, I felt like I earned it! Damn the wolves, now I have two sleep spells! That was a much more satisfying feeling than when I went from Level 22 to 23 in Throne of Bhaal, for example.
