All twenty pins are "regular digital i/o pins." They all support digitalRead(), digitalWrite(), and pinMode().
I'll go by the Arduino pin numbers, not the ATmega*8 pin numbers, just out of convenience when talking about the Arduino library API.
D0 and D1 are additionally tied to the FTDI chip and allow serial communication through the USB/serial port.
D3, D5, D6, D9, D10, D11 are additionally given access to internal comparators and timers, for PWM output with analogWrite().
D14 to D19 are additionally given access to internal analog-to-digital converters, for analog inputs with analogRead(). They are also known as A0 through A5 and it's this pin numbering scheme that analogRead() expects.