Long sleeve dresses used to mean one thing: a jersey tube you wore under a cardigan because the sleeves were an afterthought, thin and a bit see through, more decoration than coverage. That's shifted properly in the last few years. Brands finally worked out that a long sleeve isn't a bolt on, it's structural, so you're seeing proper ribbing at the cuff, sleeves cut with a bit of shape rather than a straight pipe, necklines that don't gape the second you reach for something on a high shelf. It's a better category than it was, and it's one of the few places the high street has actually improved rather than just repackaged the same dress in a new print.
The difference between a dress that earns its spot and one that ends up at the back of the rail is mostly in details you only notice once you're wearing it. Fabric weight matters more than anything, too thin and the sleeves ride up and bunch at the elbow all day, too heavy and it fights you in a warm room. A good one has some stretch in it, not just in the body but through the shoulder and underarm, because that's where cheap versions split a seam first. Check the cuff finish too, a dress with a plain hemmed sleeve end nearly always looks less expensive than one with a slight rib or fold back detail, even when the fabric underneath is identical. Hardware rarely comes into it unless there's a button placket or a tie, in which case go for anything other than plastic that looks like it'll snap on the second wear. For occasions, this is an all day dress more than an evening one, good for work, for lunches that run long, for any day between autumn and early spring where you need sleeves but can't be bothered with a jumper over something sleeveless. Colour wise, stick with ones you already own trousers and shoes for, a deep green or a plain black will outlast any seasonal print, and a rust or chocolate brown is easier to dress up or down than you'd expect. If you're starting from nothing, go for a simple midi in a solid colour with a slightly fitted waist, because that one layers under a coat, works with boots or trainers, and doesn't date the way a trend print will by next spring.
Reach for it on the mornings you can't face choosing separates, or the evenings where you need to look like you tried without actually trying.