A sleeveless dress is the one that carries heat without carrying fabric, the dress that lets you get through a warm day or a stuffy room without a sleeve sticking to your arm. That is its whole job, and on a frame like mine it either does that job cleanly or it shows every inch of forearm I never asked it to show.
If you own none, start with something in a firm cotton or a ponte rather than anything slippery, because a sleeveless shape depends entirely on the armhole sitting still, and thin jersey has a way of gaping at the side of the bust the moment you reach for something on a high shelf. Go for a shift or a column before you go for anything fitted at the waist with a full skirt, because the simpler shapes are more forgiving if the armhole runs slightly big, which on a tall frame it usually does, since most ranges cut the hole for a shorter ribcage and I end up with a sliver of bra visible at the side that no amount of tugging fixes. Check the strap width too. Anything under an inch and a half tends to dig on me by the second hour, and I've learnt to feel for that in the changing room rather than hope it settles.
A sleeveless dress earns its real value in what you put around it, not in what it does alone. A cardigan or a light jacket turns it into something for a cooler evening, a denim jacket makes it look like you dressed on purpose rather than by elimination, and a pair of flat boots in autumn will carry a summer dress another two months without anyone noticing it's out of season. For me the length check matters as much as the armhole. A dress that hits mid calf on the model hits my shin, so I measure from the natural waist down before I buy anything described as midi, because midi on the hanger and midi on a six foot one body are two different garments entirely. Watch the shoulder seam too, since a sleeveless dress with no sleeve to hide it will show if that seam sits too far down the arm, which reads as borrowed rather than fitted.
Where it struggles is obvious enough. Office air conditioning turns a sleeveless dress into a dress you need a layer for by eleven in the morning, and a proper cold snap makes the whole thing pointless regardless of how good the cut is. It is a warm weather piece, and no amount of styling turns it into anything else.