The mistake is buying a strappy dress for the first warm weekend and assuming the straps themselves will hold everything in place. They won't. A strap is not a bra, and the first time you reach up to get something off a high shelf, you'll find out exactly how much faith you put in two thin bits of fabric. The dress that looked fine standing still in the changing room can turn into a very different proposition the moment your arms move.
There are days this dress simply isn't the answer. Anything involving a lot of bending, carrying, or sitting on plastic chairs at an outdoor event wants a bit more structure than straps give you, and a windy day will test a loose, light fabric in ways you don't want to find out about in public. It's also not forgiving if you've gone up or down a size recently, because there's nowhere for a strappy dress to hide; it either sits right on you or it doesn't, no belt or jacket is rescuing a bodice that gapes. And it clashes badly with anything else demanding attention on top, a statement necklace fighting with already-bare shoulders just looks busy, and a bra with visible straps or an obvious seam under thin fabric undoes the whole point of the dress in about two seconds.
The difference between a good one and a bad one shows up fast. Cheap versions use straps cut too thin and attached with a single row of stitching that pulls at the seam within a few wears, and the fabric itself is often a flat, shiny synthetic that creases the second you sit down and clings in all the wrong places by the afternoon. A well made strappy dress has straps with a bit of width to them, properly reinforced where they meet the bodice, often adjustable so you can actually get the fit right instead of hoping. The fabric has some weight and drape to it, enough that it skims rather than sticks, and the lining, where there is one, is cut to match rather than bunching up underneath. Check the back too; a dress that's beautifully finished at the front and left raw-edged at the back is telling you something about where the money went.
Where it earns its spot is in layering up rather than down. A linen shirt worn open over the top for daytime, a cropped cardigan for an evening that's cooled off, or a slip underneath if the fabric's thin enough to need it. Flat sandals make it feel unfussy, a low block heel makes it feel like you tried. I'd start with a dress in a deep plum or bottle green, colours that don't show every mark and don't need much else doing to them to look finished.