The K-3 has a significantly redesigned body compared with earlier Pentax dSLRs. Cosmetically, it has a lot more bevels and angles than previous models. It feels very well constructed -- like most of Pentax's dSLRs, it's weather sealed -- and is relatively heavy, with a more pronounced grip.
Pentax modernizes its design with the now-common switch to toggle between movie and Live View modes with a dedicated record button. The navigation buttons have also been redesigned; pressing on the pairs of raised edges allows for diagonal navigation when relevant (like selecting focus points). The inset areas that you use for accessing the frequently used settings -- drive mode, white balance, flash, and Custom Image -- feel a little mushy and flat, but the raised sides help provide the necessary tactile feedback.
Pentax adds a locking mechanism to the mode dial, but it seems a somewhat convoluted solution. There's a lock switch you engage first, then you override it by pushing down on the center button while turning the dial. In contrast, most manufacturers simply lock the dial and you press to change it, or there's Olympus' solution on the OM-D E-M1, where you press the button to lock it and release it to change settings. The K-3 also gets a welcome three user-settings slots on the mode dial, compared with the single-slot on the K-5 II, although the K-5 II let you configure up to five different settings groups.
The K-3 is the first camera I've seen to incorporate a USB 3.0. In theory, that could mean faster download and better tethered shooting performance, though it depends upon implementation and connected devices.
That rather odd bulge on the right side is where Pentax placed the headphone jack. The continuous-autofocus setting has been dropped from the AF mode switch and now gets accessed via the info screen, a change I'm not thrilled with. The AF mode button lets you select from center spot, flexible spot, and a new 9- or 27-zone selection mode.