Bluetooth

Typically you may find usb Or optical input, sometimes both, and rca preamp output. These are basic features.

However, you may want balanced XLR input, sometimes you'll find all of the above.. XLR and RCA and coax and optical, Bluetooth (and what version matters for use case, range used, reliable connectivity, ear bud compatability, ... ie BT v4.0 to 5.2, LDAC, apt-x, etc), 3.5mm aux input, location of power and input selects for convenience sake, how easy the selects are to read and use, switches or buttons, tactility, the placement, feel and dampening on the volume knob perhaps, how easy it is to read said volume, overall build quality, remote control, digital display, HDMI input, custom EQ, 5 band, 10 band, Parametric EQ, MQA, or if you don't want MQA - none (and you may save 50-150 doing so), or ability to turn MQA on and off at will (rare), etc, etc.

The possible feature options or combinations thereof are many.