For me, simple applies to the overall look first, but also to a degree to the individual garments. A simple outfit would involve clean lines, minimal obvious detailing in the garments themselves (though there can be some), few parts, not much in the way of accessories. It would probably be quite limited also in pattern.
It could be low or high contrast, (i.e. monochrome, tone on tone, or a graphic contrast like black and white), and colour could be anything, but there probably wouldn't be much pattern, and there certainly wouldn't be a lot added to the basic pieces, whatever those might be. So - pants, top, jacket. Dress with jacket. No: added scarves, multiple visible layers, piles of baubles or even multiple large ones. Streamlined, I guess.
So I do agree with Denise in a sense. If you include too many items with obvious detailing or accessories plus highly detailed items, you are no longer in "simple" territory, although you are possibly still in "classic" and "playful" territory. (And often you are in "practical" territory, too -- a lot of people have commented on my winter looks being more ALGO in some cases -- well, I am sorry, but where we live, layers and scarfs are a necessity at this time of year!)
I'm not sure playful is as easy for me to define and it might be different according to the wearer. For one person, "playful" might involve interesting outfit juxtapositions. For another, it might be unexpected colour combinations. Still another might look playful wearing the odd vintage item, or an unusual hairstyle or Geek chic specs or a shorter skirt. Lots of possibilities.
In fact, maybe too many to make it a particularly useful style descriptor? Hmm. I wonder.
Although if it is useful to YOU and helps you find the items (and create the outfits) that you want, that is more than good enough!