The Canon EOS 1D Mark IV was built quite similarly to the 1Ds Mark III, and while the 1D X retains most of what's come before, it expands on the series' designs with a few neat refinements. The most obvious of which will be the improvements made to shooting with the vertical grip, bringing the experience on par with shooting in the horizontal orientation.
If you take a close look, you'll notice that there are not one, but two joysticks on the back of the 1D X. The new one is right below the wheel dial, and within comfortable distance of your shooting hand when holding the camera vertically. In front of the camera are not one, but two pairs of custom buttons, each also aligned for use in either vertical or horizontal orientation.
Other changes include the addition of a White Balance button to the row of controls above the top LCD, and the switching of what was previously the FEL (Flash Exposure Lock) button to a custom Function button, which makes a lot more sense for shooters who never used FEL. A new Multi-function Lock is located as an option on the On/Off switch and can be programmed to lock any one or more of the three main controls; the main (top) dial, quick control dial (rear wheel dial), multi-controller (joystick). It's a great option to have when you have your settings dialed in and don't want to switch them accidentally in the midst of shooting (stuff happens...).
Another welcome addition is a secondary CF card slot, where previously the 1D Mark IV had a CF and SD card slot. It's more economical, especially when compared to the Nikon D4 which forces you to be an early adopter of the still-uncommon XQD memory card. To be fair, we sorely missed the button illumination from the D4, which made it easier to hunt for controls in the dark.
Otherwise, the 1D X retains most of the same controls and layout as its predecessor, which means that it should be familiar to anyone who's handled one. There really isn't much else to say except that the 1D X handles really well.
One of the reasons why you don't see hordes of people carrying around a camera like this (besides its sky-high price) is how heavy it is. The 1D X is slightly bigger and heavier than the 1D Mark IV, and its weight is easily felt the moment you start carrying it around. It feels dense, well-made and able to take a few knocks, being dust and drip-proof, with a magnesium alloy body shell and internal structure. You can watch one dropping into a tree when its helicopter mount fails, in the snow, and surviving the crash.
One last minor thing: In place of a "D", you get a big, bold "X" on the front of the camera body. You'll either find that incredibly cheesy or really cool (really cool for us).