When MMOG developers attempt to create a "balanced" game, this means that they are trying to create a finite set of class/ability combinations that are - essentially - equal. Basically, it means that at a certain stage, Class 1 can avoid X damage; class 2 can block X damage; class 3 can heal X damage; and class 4 has armor that prevents X damage.
The problem is, this balance formula, one way or another, makes all the classes, especially from game to game, the same. This is why there is nothing new under the sun in MMOGs, and why sci-fi or gothic MMOGs have the same fundamental feel as any other. They are all designed with grouping and end-game raiding in mind, meaning that each class must be designed to fill a role and balanced to be the relative equal of any other character within a range of levels.
Of course, you have to balance if you're going to include PvP or coerced grouping; balance is broken in such games when it is discovered that a particular class/ability combination results in a character that is significantly more powerful than (1) other characters at its level, or (2) in group or raid situations. In a raid, all characters must be able to contribute X value, or else one is gimped and the other is over-powered.
But, what if the developer threw out the whole concept of "balanced" groups and forumlaic end-game raiding? What if PvP had no guarantees that your character would be the relative equal of others with similar time in the game? What if there were not one or two ways to create an over-powered or gimped character, but virtually infinite ways? What if one of the accepted principles of the game was that you could create a character overpowered in some ways and gimped in others, instead of a development team that slow-fed you a sugar-coated diet of forced balance and end-game relevance?
From the perspective of what this blog is about (a 24/7 progression game whether online or not), a player could make themselves, say, a fire tank by putting virtually all of their advancement time into fire resistance, generating aggro, and health. Such a character might be able to handle fire-based enemies well beyond the capacity of others of equal time investment. Imagine a much higher-level (meaning: more time invested, since this blog is advocating a game without levels) fire mage attacking a much lower-level player who has their time invested thusly; the lower character might be impossible for that particular higher-level character to kill.
But then, an ice-mage would make short work of that same character.
Imagine a game where you can pursue highly specified characteristics indefinitely. The ultimate fire-demon conjurer. The ultimate two-handed mace expert. The ultimate healer of humans. The ultimate healer of mixed groups. The ultimate chain-lightning caster. Near-infinite specificity of ability and no limitatation to how good one can get at it. It might be that you have virtually no health, no defense, no crafting capacity or any other game skills whatsoever, but if a group can smuggle you into the enemy camp alive and get you to just cast one ultimate chain-lightning spell, then even if you get killed immediatly afterward they have a chance of winning because you've decimated the opposing army.
That's real variety, and endless potential for highly individual characters. If it is in a system that has 24/7 advancement for all your characters, then even if you find one of your characters gimped from doing something you wish to do, all you have to do is set them on a development course to fix the problem.