As one progresses through the game the frequency at which one levels up slows to an intolerable crawl. This factor conspires with several others (such as inventory management) to make the game become progressively more dull as one plays through a game. I understand that this is something that occurs in many RPGs but I see no reason why you should seek to emulate the extremely poor design of other games. Especially since the issue could be corrected, both here and elsewhere, simply by increasing the xp values of higher level monsters.
Easier than that, changing the XP requirements. But I'm really not sure this is such poor design; the monsters don't become markedly more difficult between floor 13 and 14, while between floor 1 and 2 or between 2 and 3 the difficulty jumps.
I agree that it's a problem; I've yet to kill Dredmor, but probably a dozen of my characters have "died of boredom" around dungeon level 5-8, i.e. I saved them to try another build that might be less tedious, and never came back to them. I don't know what the solution is, though...I do appreciate the difficulty of balancing a game, especially one with as much going on as DOD has. I don't think dumping a bucket of XP on the lower levels will be the whole answer.
Make it so enemies in later floors get more dangerous (hehe... like those bloody arc dragons in shiren the wanderer that breathe fire through walls, and ghosts (again, like in STW) that move through walls!!), but make their drops and EXP better, as Bohandas said earlier.