Eltab
Lord of the Hidden Layer
The limits can help provide flavor to THIS world, compared to all the others. Consider Dark Sun vs Forgotten Realms.
I would put limits on subclasses not whole classes ("Everybody knows that Gnomes are the only really good Illusion Wizards, but they are horrible Greatweapon Fighters; they can barely pick up a two-handed anything!").
Ideal candidate subclasses for racial limits would reference / enhance the racial abilities in some way: got Darkvision? then Night Ward Ranger is for you.
And if in print for public consumption, an obligatory short sidebar about "Ask your DM."
I would put limits on subclasses not whole classes ("Everybody knows that Gnomes are the only really good Illusion Wizards, but they are horrible Greatweapon Fighters; they can barely pick up a two-handed anything!").
Ideal candidate subclasses for racial limits would reference / enhance the racial abilities in some way: got Darkvision? then Night Ward Ranger is for you.
And if in print for public consumption, an obligatory short sidebar about "Ask your DM."