Diodes and switching frequency
Current capability (which will depend on temperature), reverse recovery time, forward voltage at the expected currents, reverse leakage (heavily dependent on temperature). PIV (Peak Inverse Voltage) rating. 

Generally you'd use a Schottky diode for 50kHz unless it was relatively high voltage, then you'd use an ultra-fast recovery silicon diode.  

The considerations are similar regardless of voltage or current, but the parts will be quite different for a 5V/1A circuit vs. a 500V/0.1A or 2V/100A circuit (in the latter case, you'd probably want to use a synchronous rectifier and eschew the diode entirely).