Not sure if I understand the question...it would probably be much quicker to list the commonalities than the differences.
I think the point is the hard stuff, given that the styling and the touchy-feely stuff is pretty obvious.
Ill say this: even shared parts aren't necessarily "the same" thing. Take the 8hp for example, the shifting logic is different (think about having two equally built basketball players, one is KB, the other not: the difference is in the brain). Otherwise a 300,a Bentley, a BMW, etc would all shift identically; but that's not the case.
Materials, are different; even apparently silly stuff like the dashboard support is made of magnesium in order to save weight.
The actual architecture is different (not talking platform here, I bet you'll be able to build a Ghibli and a 300 in the future on the same line...technically, that is).
The tolerances are worlds apart (according to a tester some of the tolerances on the new QP and Ghibli exceed those of some Ferrari models).
Suspension is different; not all wishbones or multilink are the same. Take this pic of the 2009qp with the Maserati Low stacked Wishbone
http://www.midwestscc.com/forum/attachment?id=2504
And compare it tot the one from a 300 (Mercedes S floating arm, or high wishbone suspension)
Ghibli has frameless windows, different steering mechanism, a different optional AWD system, a different exaust system...
The engines are different. Even the V6 with its shared block is not comparable to a Pentastar.
In short: are they ENTIRELY different? No, they share some components (less than 10% is what I heard); but they are very different cars (today). They spent a ton of money (some say they went overboard) at Maserati for the development of the QP, exactly because they couldn't take an SRT300/charger and reskin it (they did test that option early on, and decided it wouldn't work) and also because they didn't want to compromise on some aspects.
I think the renewed Challenger will be the first to sport some of the changes coming to Chrysler products. There we will see how much the new stuff differs from what we have today.