For years I figured modern wall art meant one big canvas, something abstract and gray, hung dead center over the sofa because that's what the showroom did. What changed my mind was a bare stretch of wall in a rented place with old plaster that I wasn't allowed to patch more than once, and a piece that finally made that wall feel finished instead of just covered. Scale did that, not the art itself.
Measuring up matters more than people admit. The common mistake is buying something sized for a gallery wall and hanging it like a postage stamp over a long sofa, where it just floats there looking apologetic. A good rule is to let the piece or the grouping take up roughly two thirds the width of the furniture under it, and to hang the center around fifty seven to sixty inches from the floor, eye level for someone standing, not someone six foot four leaning on the doorframe. In a stairwell or a hallway, you're working with a different problem, usually too much height and not enough width, so a vertical piece or a tall, narrow grouping reads better than one square frame marooned in the middle of a wall.
When you're looking at it in person or it shows up at your door, check the depth of the frame against the wall it's going on, because a deep floating frame against a wall with an outlet or a light switch nearby can look crowded fast. Check how the piece is meant to hang, French cleat, wire, or sawtooth, because a heavy piece on a flimsy sawtooth hook will slide crooked within a month, and run a hand along the edges for any warping in the frame, which happens more than you'd expect with humidity, especially down here. If it's framed behind glass, look for any flex when you press gently in the middle, that's a sign the backing is thin and will bow in a year.
If someone's starting from nothing, I'd tell them to buy one piece they actually like looking at, bigger than they think they need, in a frame that can take some bumping around, rather than three small ones they're lukewarm on. Moving house is the real test. Cheap frames crack at the corners in a box, mats yellow if they were ever lower quality to begin with, and anything with real glass earns its keep by surviving that trip intact.
Good modern pieces don't really age the way an old painting does, they don't gather that kind of history, but the ones worth keeping settle into a room. The color fades a touch if it's near a window, the frame picks up a scuff or two, and somehow that makes it look more like yours and less like it just arrived.