I think the big flaw with his design, and with tabletop RPGs nowadays in general, is character attachment. More often than not, in my experience players come into a campaign with the idea and intent of carrying their special little snowflake all the way through. And they expect you as the GM to facilitate this. The problem with this is that, on some base level, it totally guts stakes and danger.
Let's say you buy into Invisible Sun, get a group together, bullshit some encounters together. And your players spend all this time coming up with their wacky powerful dream characters. And the first session in, someone does something stupid and gets wrecked by a phase beast. Or tries to jump across a chasm of fire and fails. Whatever. That player is going to feel cheated out of all the time he spent on crafting his four-armed smokestack-for-a-head sorcerer or whoever. He won't be motivated to go through all that hard work again, and really I don't think he should have to.
In my experience, modern players turn their characters into creative fetishes. And modern systems are designed to facilitate this. Anyone who's played AD&D will know this is NOT how campaigns back then were run. Players don't know it -- and more often than not I think they'll perish the thought -- but this is why it can be so hard to keep gaming groups these days going. Because secretly, nobody but the GM REALLY wants to be there. Even if they don't know it themselves.
I could have sworn Monte was involved in Planescape.
