We're in a habit of thinking of a class as being "stuff you can do". Some designers tend to pile on extra stuff you can do, and maybe (if we're lucky) some limitations on the class to balance it out. But not the kind of limitation that makes things happen. For example, ability score minimums, alignment restrictions. Or, worse, class grinding, like the 1e bard.
What I'd rather see are limitations that cause potential trouble for the character, or at least cause things to happen in the character's presence that might have to be dealt with. Consider first the basic four classes:
- Fighter abilities tend to put them directly in the path of risk. Mostly, they are as good as their weapons and armor.
- Magic-User spells can enable them to avoid risk a couple times, but they also embroil M-Us in the spell economy, forcing them to look for more spells or magic items and protect the stuff they have.
- Clerics have some of the same issues as Fighters and Magic-Users, but also thy are expected to confront the undead, which isn't guaranteed to work.
- Thieves can disarm traps -- or accidentally trigger them; they can pick pockets or sneak around -- or get caught doing so; they can decipher treasure maps -- or get them completely wrong.