A GM and/or player with no sense of scale? Somebody performing a futile rules exercise in statting up a character that might be able to take on an entire plane of existence at once.
Y'know, maybe 2e had some right ideas, like mortal progression stopping at 30th level (DM's Option: High Level Campaigns & Dragon Kings) and gods only being able to go to 40th level (Faith & Avatars/Powers & Pantheons/Demihuman Deities).
Maybe given how multiclassing works in 3e increase that to 40th level (for mortals), but still, a hard limit on progression seems like it might fit for my views on D&D, there comes a point where you really do hit the limit of what really are the limits of mortal capacity, a point where you reallly have pushed the mortal body and mind to its limits and only some sort of transcendental experience (like divine ascention) lets you level up any further instead of sticking your "XP straw" into yet another monster and sucking out it's valuable XP like a gamer gulping down mountain dew.