Una, I think you are psychic...I was just thinking about something very much along these lines tonight at dinner. I was thinking about favorite wardrobe items rather than workhorses, but the two are often one and the same thing.
I liked Sunee's list, and she included a lot of things that I had pinpointed when I was musing over my bland vittles. Here's what I came up with:
-Versatile (my workhorses are neutral colors and classic cuts and adaptable for a wide range of situations)
-Well fitting
-Warm for winter, cool for summer (i.e., they work well with the season for which they are intended)
-Good quality
-Comfortable
I will say that my overall style has a good dose of modern classic in it, so that is why classic cuts are workhorses for me--I can pair them with just about anything. But your style is so different from mine. Perhaps for you, a workhorse would be a black, edgy piece that is a more on the "toned-down" side and could be easily paired with a variety of items.
I will also say that price isn't much of a factor for me. In fact, the more expensive an item is (like my Aquatalia boots), the more I try to wear it, as long as I'm not going into clearly dangerous territory (like inches of muddy water).
I will also say that I had to come up with this list because I am like you--I have a hard time pinpointing what's going to be a workhorse. Some of the things I purchased intending to be workhorses (the Aquatalias) became so; other things I purchased to wear once in a while became unexpected workhorses. I think that one of the main reasons expected workhorses don't pan out is the "comfort and fuss" factor, which is often something that takes real-life wearing to figure out. Does this sweater remain comfortable throughout the day? Do I have to keep adjusting the waistband on these pants? etc. So you can't always know. On the other hand, I think that unexpected workhorses will be created any time all of the above criteria that I listed are met, simply because the item works with A LOT in your wardrobe and is therefore an easy thing to reach for when you're getting dressed.