Wow, what a great question, and yes, I will take each of these items, please! Well, maybe not the boiled wool coat, if I am paying attention to my own climate...but otherwise...
Love the booties especially.
Like Bonnie, I like the "will I wear this 30 times" question. For classic dressers like us, it works especially well; of course some items need to be excepted from the mantra, but I figure that's okay because workhorse items get at least double that wear and often five times that amount.
As to what makes a workhorse, I really like what Approprio (Liz) said -- brilliant as always: "It's something that completes an outfit formula with panache. Not exactly a statement piece, more like a particularly stylish solution."
For me, that is always, always the case. If I think back to my own surprise workhorses, they all met that description. The thing is, sometimes I didn't know I had a hole until I filled it, and then -- bingo!
Over the last few years, surprise workhorses include the items below.
I like Liz's word "solution." Because it's the items that answer some dimly felt outfit "problem" that turn into winners. So for me...I needed (without knowing it) a large bag that would fit my computer yet could feel somewhat luxurious. (Minkoff hobo). I needed an easy to carry and rain-worthy but still stylish bag for gym and other super casual outings. (Zara backpack). I needed to amp up the shine in my outfits (watch, silver sneakers). And so on.