One of the themes of Planescape, of course, is that there is ALWAYS a bigger fish. Between all the demon princes and archdevils and Powers and Not-Quite-Powers running around, sometimes it's hard to remember that mortal adventurers can do anything at all. But every setting, it seems to me, has a point at which the PCs graduate from run-of-the-mill adventurer to something that people recognize as above average. In Eberron, it might be 6th-8th level, in some especially gritty games maybe even 5th level is impressive.
I want there to be a point in Planescape where the PCs are recognized, maybe not as historically terribly important (given the nature of "history"), but as significant figures of personal power in their time. Obviously, it is not a strict correlation with level, but rather something that will come from their exploits, but it's hard to gauge what's an impressive accomplishment in terms of Planescape. Do people recognize your name after you've been to Hell once and survived? What about if you defeated a cornugon? A pit fiend? What does it take to gain some level of fame or notoriety, and what is considered relatively standard for adventurers?
My hunch is that the kind of adventures PCs can succeed in around 15th-16th level are the kinds of things that can impress even Planescape residents, because while this shouldn't be strictly tied to level, well, you can do bigger and better things at higher levels. Does that sound roughly right, or can PCs stand out before then (even besides those occasional truly phenomenal stories)? Or is it the case that the PCs will never be recognized as exceptional outside of their personal circles, because the Great Wheel is just too big? Thoughts?
I'd go with the planes just being too big of a place for the PCs to have much of an impact outside of their personal circle. Too many people, too large of an area, and planar cynicism seems to be a power unto itself. Even if the PCs manage to accomplish some amazing feats, the kind of reaction it's likely to generate is "Yeah yeah, I know twelve other berks who did pretty much what you did. You're amazing, just like them and every other basher who's done it."