What are the important considerations when choosing a pass transistor and bias resistor for a buck regulator?
Some background: Switching supplies are good because the switching transistors are, ideally, at zero current while off and they are at zero voltage while on. Both times, one of the numbers for V or I in the equation V*I=P  are zero, making the transistor losses P also zero.

The transistor will have low conduction ("on") losses if it has a low saturation voltage VCE,sat; we can't really get V=0, but we can get close to this design goal. (V*I=P will be low because V will almost be at zero.)

While the transistor switches from on to off or from off to on, neither V or I will be close to zero. Thus, you want to keep the switching time as short as reasonably possible. For the transistor, this means: It will have low switching losses if it turns on and off fast. Generally a problem with bipolar transistors, but you might want to look for transistors designed for switching applications; these ususally have a comparatively high transition frequency ft.

The resistor between base and ground actually is not a bias resistor but a pull-down resistor. The IC can only pull the base high (i.e. bring the external transistor to conduction). Once the IC stops pulling high (= stops providing base drive current), it is the pull down resistor's job to bring and keep the base low and keep the transistor off. Keeping it off is not so much of a problem as bringing it there. The lower the resistor's value, the quicker the transistor will be brought from its on-state to its off-state, minimizing the losses during each turn-off process. Then again, if the resistor is too low, it will demand and waste extra current from the IC's switching pin (#2) while on.