The one thing, the biggest, major thing, that I would like to see improved is lighting and the way Poser deals with shadows. I totally hate the shadowcams and most users don't have a clue how to start with those things.
I'd love to seem them gone or their usage improved.

Other than that - as I said before, it sounds to me like the OP wants D|S, not Poser. No extra rooms, no "bloatedness", an image layering thingy, a uh... what was it? Image editor, right? (I don't use D|S it doesn't run on my box, so forgive me if I don't know what's in it.)

It sorta makes me giggle tho...
I don't think anyone using Max or Lightwave would be caught DEAD using an image editor from within the program.
And as someone already pointed out - you don't need a layering program. That's what nodes are there for, but yeah, they aren't always simple to use. But then, if it's simple to use, that means you have less options.
I'll keep the material room and my shader nodes, thanks. :)

Silke