The output simulation used in both simulation and design exercises is stupidly implemented (idiot me!). If a circuit has feedback loops, it will simulate forever.
The exercises currently assume only one output of the circuit. This is limiting in the kind of components and circuits can be used. For example, the existing half-adder and half-substractors cannot be properly used.
Most of the components (except output and NOT) should support (almost) arbitrary number of inputs instead of the currently fixed two inputs. Also, the editor should allow changing the number of inputs.