The pocket on the thigh that looks useful and then eats your phone, your keys and the receipt you needed, until you're patting yourself down like you've been stopped by security. That's the thing nobody mentions before you buy your first pair. And yet I still reach for mine more than almost anything else I own in summer, because nothing else lets you cycle, sit on grass, or carry a water bottle without a bag.
They sit well against a plain fitted top, which sounds obvious but matters, because the shorts are already doing a lot of visual work with the pockets and the stitching and the toggles at the hem, so anything busy on top turns the whole outfit into a shouting match. A simple vest or a fine knit tucks in nicely. A denim jacket over the top on a cooler evening does the job without fighting the utility feel. Trainers suit them better than sandals, in my experience, there's something about the shape that wants a bit of structure underneath, though I know plenty of women who'd disagree and wear flat sandals happily all summer.
If you've got none and you're starting with one pair, go for a mid length in a neutral like khaki or stone rather than black or camo. It sits better with more of what you already own, and you're not trying to match a pattern to anything. Knee length or just above is the easiest length to start with too, long enough to feel sensible, short enough that you're not wading through fabric in the heat.
The difference between a good pair and a disappointing one is almost entirely in the fabric and the closures. Pick them up and scrunch a handful in your fist. A heavier cotton or cotton blend will hold a crease and spring back, a thin one stays crumpled and looks tired within an hour of wear. Check the pocket flaps lie flat when empty rather than gaping, and give the zips and poppers a proper tug, cheap hardware catches or sticks within a few wears. Turn the hem up and look at the stitching, double rows last, single rows fray at the first wash. And have a feel of the waistband, elasticated panels at the sides save you a lot of bother on a day when lunch was bigger than planned.
The one mistake to avoid is buying them too fitted through the thigh. Cargo shorts are meant to have room to move and pockets that actually hold things, and a slim cut defeats the entire point of owning them.