typically seams (you want them to be as seamless as possible, no rough stitching!), chamois quality (it's more than just "padding" - a good synthetic chamois is designed to be skin-like and to function as an extension of your body), and elastic quality (leg gripping elastics that wear out prematurely are a bummer).

i've never used the performance house brand, but i've had not-so-stellar experiences with lower-end pearl izumi shorts. i have a nicer pair that i like a lot. assos are top of the line, but insanely overpriced. giordana, craft, hincapie, nike, descente and others make good shorts.

for long and hot rides, as gross as it sounds, chamois cream-type products (aka "butt butter") are fantastic. at first, it's like sliding your junk down into a pool of cold vaseline, but it helps you move and keeps you cool.

and just in case you don't know, never wear anything under cycling shorts! chamois pads are designed to be used right up against the skin.