The problem with this one is that the modeler failed to keep the same design aesthetic between the "D" part and the ring. It looks tacked on.
Granted, but assuming the ring is - as would make sense for, you know, it basically being the nacelle - not inhabited, there's little continuity to keep outside of the decals (which carried over) and windows (which would make no sense). I agree it looks a bit too blank, but... really, what'd be there to add for visual continuity?
edit: forgot my quote.
I think he's also noticing it's got a simple rectangular cross-section with straight walls, which doesn't go well with the D's sweeping organic curves.
EDIT: I should read the last post before I post!