I've talked a little more about characters with powers than I intended to, but there was kind of a side issue peeking out of the comments to the last post about a super-man character vs. a super-fighter. 1d30 presented an alternate character class which I could see using in a high-power campaign with just two classes: a super-fighter and a super-mage. But I wouldn't want a player to choose a super-fighter class in a game centered on the standard classes, even though I think I'm pretty lenient with regards to substitutions. Why not?
I expressed it in the comments as creating a variant class by trading an existing power for the replacement power. Basing a super-man class on the Cleric, but upping the xp requirements to those of the Magic-User and swapping out the Cleric powers for a very limited selection of at-will powers seems reasonable to me. So would replacing a standard Fighter's multiple attacks ability with a single, scaled power.But adding several powers to the existing Fighter without really giving up much in return feels like it goes too far.
Not all unique character concepts are equal, in my opinion. There's a subtle difference in intent between wanting to play a unique concept because you like characters who can fly, or characters who are invisile, and wanting to play a unique concept because you just want to be better. If a player says, "I want to play a pixie!" in a standard low-power D&D fantasy setting, I think "Oh, that player likes pixies, or likes flying characters or fairy characters in general." If a player says. "I want to play Superman!" I'm a little more worried. Is it because of a love of Superman, or a fear of failure? If it's love, we can talk about how to get close to what the player wants; maybe start out at low power or up the xp requirements or both. If it's fear, negotiations like that tend to bog down; players who are afraid of feeling ordinary are unlikely to accept any limitations on their character concept. They will likely go on to write "new" character classes which are really just the same as the base classes, but lots of free extras to make the player feel amazing. They smack of munchkinism.
To put it in a more ridiculous way: is the player chasing a schtick, or chasing a carrot?
Context is everything. I think 1d30 was proposing the super-fighter in the context of replacing more mundane classes. If you establish that everyone's going to play at a higher power level and there's going to be lots of flash, bang, and pow, then Superman might not be out of context. If you establish that it's going to be more of a low-fantasy, sandbox-y, Fafhrd and the Grey Mouser-style game, then Superman with full powers at 1st level kind of sticks out.