For years I treated t-shirt dresses as the thing you wear when you've given up, the fashion equivalent of a shrug. Then I wore one on a day that involved three buses, a late lunch and an argument with a parking meter, and it looked exactly as good at the end as it did at the start. That's when I stopped sneering at them.
They're not clever about everything, mind. If you want shape, a defined waist, anything structured, this isn't it, and on some bodies the straight cut reads as a bigger t shirt rather than a dress, which is a different outfit entirely. Thin jersey can cling in ways you don't notice until you catch a shop window reflection and have a small internal crisis. And they're not forgiving of a bad bra day, there's nowhere for anything to hide, so whatever's going on underneath is very much the headline.
Colour wise, I've gone off black for this one specifically, it can look like loungewear rather than a dress once the light catches it wrong. A proper rust, a deep teal, even a plain oatmeal, all read as more considered without trying harder. Stripes work if they're not too fine, fine stripes on thin jersey have a habit of doing something odd around the stomach that nobody asked for. Busy prints are a gamble, the shape is already simple, so a loud print can tip the whole thing into looking like sleepwear you've accidentally left the house in.
How they age depends entirely on the fabric weight, and this is where I've been caught out twice. A thin cotton blend goes see through at the shoulders after about a dozen washes, you can genuinely watch it happen. Anything heavier, a proper slub cotton or a thicker jersey, keeps its shape for a full season of actual wear, bag included, and only really gives up at the neckline, where it stretches out if you're the sort of person who tugs things over your head in a hurry. Wash cold, skip the tumble dryer unless you enjoy dresses that fit your nine year old self, and turn it inside out if there's any print on it at all, I learned that one the hard way with a faded logo that now looks like a ghost.
Next time you're holding one up, check the fabric weight first, pinch it between your fingers, if you can see your hand through it in the shop, you'll see a lot more than that by the summer.