I answer a bit late in this topic. I'll try and answer to everything at once...
I agree that Core themes should be kept minimalistic, so that Core remains Tiny
The current balance is good in my opinion: rather plain, but with a logo and wbar so as not to be too
plain. And I like the logo and the background.
Please no bootsplash! Or as a separate (additional and facultative) initramfs.gz.
I don't think 3D themes are a good idea: they add bevels, borders, and so on, and waste screen estate, in this day and age when we have smaller screens than we used too (mobile computing). Besides, other desktop environments and web sites are shifting to flat themes, away from 3D.
I don't have an opinion on the icons, and I usually just use Tango, although I like to try something else once in a while.
From the window managers that are supported by TC's onboot/ondemand system, I prefer Fluxbox, but then, I like the default red+gray theme (I don't remember the name) well enough: it's small and nice looking.
I think GTK should be prefered over Qt, not because it is better, but because from recent tests I can tell that GTK apps tend to use less memory. On the other hand, the current unfinished shift to Gtk3 makes it mandatory to have both Gtk2 and Gtk3 (but the same could be told about Qt, with Qt5 coming).
@Misalf: I don't agree that Compiz effects are only resources-wasting eye-candy. I find many of them extremely productive! For example, I use the window transparency a lot, to do things in a window while following instructions in another window (bellow). I also use a lot the "exposé", and the zoom-in/out of windows, and the tabs!
I'm not much of an artist, and I'll leave that to others. In short, my opinion is that a good balance has to be found so that Core 5 remains tiny; I agree with Misalf on this.
And the "toolbox" idea is good.