There are two good reasons why Fujifilm's X-E1 is an improvement on the stalwart XPro-1: size and speed.
Most of the photographers on the New Zealand PODAS used a second camera in addition to the medium format Phase One and one of them had the new X-E1. I was a little green because I could see that the smaller size made it that much more convenient than the XPro-1 tucked away in my camera bag.
Of course, to achieve the smaller size some sacrifices were made and the X-E1 uses an OLED EVF (electronic viewfinder) instead of the optical hybrid viewfinder as found on the XPro-1. While some photographers love the optical viewfinder, its functionality harking back to the film days and traditional rangefinder cameras, I was quite happy with the high resolution (2.36 million pixels) EVF on the X-E1.
The other issue was speed.The X-E1 has a much faster 0.1 second autofocus and while I didn't notice a big problem with the XPro-1, after pressing the button a couple of times with the X-E1 I could see that it was greatly improved.
The X-E1 also has a pop-up flash gun, but the rear LCD screen is smaller in size. Apart from that, the general feeling and specifications are much the same as the XPro-1 and no doubt the new camera will pick up more converts, as well as being attractive for those who already have the XPro-1 and some of its interchangeable lenses.