I am loving this thread. Thank you, Una!
I like Sunee's list - it's a really good start. For me, I find I gravitate towards classic construction and shape, items that will "punch it up" when worn with something very basic, and things that I don't have to worry about.
The first point is pretty straightforward - I don't really go for on trend very much, preferring to stay with shapes that aren't really era-specific. Skinny or straight leg jeans, fitted ribbed or oversized boxy sweaters, turtlenecks, volumunous maxi skirts. The second is tied in to the not going for trend thing - I do gravitate towards a quirky detail, though. Perhaps it's a boxy sweater in a pink leopard print, or a skinny jean with a bright floral pattern, or a traditional wingtip shape with black and pink contrast. I can wear the sweater with plain straight leg jeans, and be 100% comfy AND look kinda cool. The floral jeans can be the centerpiece, with a plain white button front shirt, and I look like I've made an effort.
I will say that for me, neutrals are not typically my workhorses - I treat jewel tones as neutrals, though, so raspberry pink, teal, and leopard print are my "neutrals." If it's one of those, it's almost guaranteed a regular rotation. (Obviously, things like white v-neck tees and plain jeans are a given, here, I think, so I'm thinking beyond those things to the things that might be less readily apparent)
There's a lot of talk here and on other fashion boards about buying quality over quantity, but I just... I can't. I can't do it. I've tried, and it seriously stresses me out. I'm guaranteed to chuck something in the drier without thinking, or accidentally leave a lip balm in a pocket and ruin an entire load, or get it covered in mud because one of my dogs gets too excited. I have to feel 100% ok about it going through the home laundry cycle without any special needs and/or getting completely destroyed. (It's important to note here that I have a fair amount of decent quality brands in my wardrobe, but I'm a lot more OK accidentally shrinking my 100% lambswool J. Crew sweater I purchased in a by-the-pound thrift than if I bought it via J. Crew. There's a LOT less pain for me in knowing I paid $0.78 than $78, if you see what I mean)
In my wardrobe, those are the things that get reached for the most: traditional cut/ fit, quirky but not trendy detail, and inexpensive. For me, those make workhorses.