Regular (not Zener) Diode as Voltage Regulator
The choice of voltages is very limited.  For a silicon diode it will be around 700 mV.

The voltage is not well regulated.  The forward voltage of a diode is reasonably invariant with current over some range, but it's not as good as most zeners.

The temperature dependency will be higher.  This is particularly compared to a zener roughly in the 6 to 6.5 V range.  There are two opposite temperature effects that cross over at about that voltage, reducing the overall temperature dependence.



One place where a diode as a shunt regulator can be useful is when you want to generate a voltage that offsets another diode or the B-E drop of a bipolar transistor.  In that case the temperature dependency can actually be beneficial.