For me it is the lines of the fenders (the top also plays a role, but).
When I look at the concept, the fenders are very agressive, they make me think of haunches of a great cat, a lean fast panther, haunched & starring at its prey. The flared fronts and significantly bulging rear fenders of the concept, coupled with its low stance just make me feel like it's "going to strike" - for the throat.
The rt on the other hand, with its straight edges, tame rear and significantly reduced fender lines just doesn't evoke any of those feelings for me. In person, I believe the car will be meaner looking due to its reportedly wide & low posture. But, the lacking fender bulges and hard straight lines are the wet blanket for me. I am holding my final judgement until I see on in person though.
Dave.