- Class-restricted gear. For example, you can’t use ranged weapons on any class that’s not a rogue.
- Class-restricted aspects and uniques.
- No way to access abilities and passives from other classes.
All of these things were possible in D2 by the way, more than 20 years ago. In fact, a popular build in D2 was the werebear sorceress, which is something that is simply impossible in D4 given the current design paradigm.