"We've been very closely involved in the development of the machine," said Hermen Hulst, co-founder of Killzone developer Guerrilla Games, to The Guardian. "It's no longer designed in an ivory tower somewhere in Tokyo, it's shared with us, with [Uncharted developer] Naughty Dog, with Sony San Diego – and together we've built the machine."
The Guardian posits developer consultation and discussions with publishers like Ubisoft led to the PS4 being designed in line with modern PCs, so devs could easily comprehend the hardware. Other design elements noted were greater social integration and controller elements, like concave triggers and the headphone jack.
The Guardian has far more on the roundtable design.