The tail end of yesterday's post, where I started rambling about handicapping characters with abilities that are too powerful or too numerous, got a little bit confusing, even in the comments when I tried to explain it better. So let me start that section over.
The core of what I'm getting at relates to Articles 2, 4 and 5 of the FLAILSNAILS conventions. GMs can decide how to import or convert characters from a game that is "roughly the same genre and system" as whatever they are running; they can also decide what the level limit is for a given session and how to handicap or modify characters, spells, powers, or magic items that are "overlevel".
Certain classes seem inherently overlevel to me, because they add additional unnatural powers beyond what other characters of a similar level have. I'm not talking about abilities that are mechanical implementations of what an OD&D player would assume any character could try, like a sweeping blow. I'm talking about a 1st level character who is equivalent to a 1st level fighter in all ways, but has a couple extra pseudo-magical abilities, like Darkness 15-Foot Radius or Shapeshift. I wouldn't be comfortable allowing one player to play a character way above what the other players in my campaign have available to them.
What I'm thinking is that the best way to handicap these is to interpret them as if they had changed classes. A Darkness power is basically equivalent to a 2nd level spell, which means that fighter could be explained as a 3rd level Magic-User who switched to Fighter. That's worth at least one handicap. "Low-level" characters with lots of high-level powers would have more handicaps, or one or two severe handicaps. Since we're talking FLAILSNAILS, here, the handicaps go away as soon as the player takes the character to another GM's game.