I agree that compositional objects should probably be on that list...although, to be honest, I think that nine out of then times they are (at the same time) a sub-category of the above four. Then again, that still leaves us with 10% when it isn't
As for your Full Throttle example, I'm not sure I agree. I'm not saying it's devoid of details (let alone stuff), but...well, to explain my thoughts, I recreated its line-art. As you can see, the image works perfectly well in pure black and white, and this way we can take a different look at the image, without getting distracted by colours, hues, lighting, etc.
Funnily enough, the bike makes the scene less dense, because it obfuscates most of the elements on the back-shelf (and the boxes and chests that you still see have only few details). And while the bike itself has some detail, this is not a problem, because the bike and Maureen are (together) the center-piece of the scene (this is also enforced by Maureen being animated).
Horizontal and vertical lines, as well as intersections of objects, contrast, and arrows (the shadows towards the hose e.g.) help you find all hotspots in this room (indicated in red and blue). The hue is certainly helps, but it is obvious from the line-art that it's not the main, let alone the only, property of the image that does. The objects in the room perfectly guide you towards these three items. Interestingly though, that when you enter the room, Ben will (fully or partially) cover these items: he's standing right in front of them. I think that was deliberate, because we don't want the player to see these objects just yet; we first want him to talk to Maureen.
When I was writing this post I started thinking: are Ben and I talking about the same thing when we say details? - Because to me, most of the image is not that detailed. Yes, there's a lot of stuff, and that stuff tells a story (Maureen is more of a patch-upper than a fixer), but most of it is not that detailed.
What bothers me though in this image, is the big, beautifully rendered, highly detailed thing on the left. Why does it bother me? - Because it's merely filler while it looks like a center-piece. Even worse, I can't even interact with it, even though the amount of detail tells me that I should (even if only to get a 'no' or an 'it looks complex, better not tough it'). In my memory that was one of my major gripes with this excellent game: in many rooms all these detailed elements told me two things: 1. there's a lot to see in this world, and 2. there only little to do in this game. As a result, this filler object - due to it's overly detailed rendition - pulls me out of the game-world rather than giving me something to explore. And that's why I think you should very careful when you add to much detail to filler elements (that have no pay-off for the player).
I'll end with a link to some screenshots from the upcoming Full Throttle Remastered
. There's also a high-res interpretation of Maureen's house.
EDIT: This also made me think about backgrounds and backdrops. To me a background is something the main-character can interact with, while a backdrop is just a pretty picture with a walking area. The player character is clearly part of the same world as the elements we see in the background, but a backdrop is just there for a player to look at...if that makes sense.