A lot of the factors that apply to Parkas & Jackets also apply to pants (OK - maybe not the stuff about hoods.)
The equivalent of the collar and powder skirt is the cuff. Make sure that there is an elasticised inner cuff that closes around the top of the boot to stop snow getting into your boots. Some pants do this with an elasticised outer cuff, but this looks dorky.
Pants should be high waisted enough to overlap the parka to provide a weather seal with the parka (unless you are so cool that your pants have to be half way down to your knees.)
Some pants have zippered vents on the inside/outside of the thigh and/or knee for ventilation.
Some pants have bibs to aid in keeping snow out.
Braces are great for holding your pants up, but can mean getting almost totally undressed to go to the bathroom. Particularly for women, a belt is more convenient. For blokes, a zip fly is vital. Some brands or styles of pants don't have them.
A lot of snowboard pants have extra reinforcing and waterproofing on the backside. As boarders spend a bit of time sitting on the snow this is probably a good idea.