The Watson reprap represents a significant departure from existing printer designs in that it attempts to achieve some uniquely different goals. Primarily, the Watson is designed to be rigid and scalable. Rather than taking a "barn raising" approach to build the frame which then requires numerous adjustments and measurements to achieve good perpendicularity, it relies on a single accurate component (the base plate) and builds from the ground up. Additionally it reduces frame flex and vibration through careful component relocation and mechanical design. By relocating the Y axis via. a gantry-type design, it also maximizes build envelope for a given size printer and no components move beyond its static footprint (making it more desk-friendly). Ideally it should be robust and easier to build than current designs, but likely that will take some design iteration to get right and is not the primary focus.
The Watson reprap attempts to achieve its design goals via several specific departures from existing printers.
Perhaps the most defining feature of the Watson reprap is the central baseplate. It is relatively simple given minimal tools and materials to either buy or fabricate a flat plate with good perpendicularity on the sides - whether made of wood, plastic or metal. All components are mounted off this one part and by leveraging the simple accuracy of this one component - the overall frame should be stronger, more accurate and simpler to verify than the complex, spindly truss frames of other printers. Perpendicularity in at least X & Y axes is more reliable because there are fewer components and points of variance between this plate and the motion of the axis. The potential for twist in the frame is dramatically reduced and wherever possible a focus on increasing torsional rigidity has been made to reduce "tower shake".
Several existing designs transmit the stresses of the motors through the least supported elements of the frame (ie., X & Y motion). In very small design packages, the strain induced by these motions may be acceptably small - especially at low speeds. As the size of the frame is increase and/or motor acceleration is pushed to the limit, these deflections become considerable and affect both print quality and component life. In the Watson RepRap the location of motors and the forces they transmit are specifically located so that they have the most rigidity and support in the frame.
There are disadvantages to moving all 3 axes together vs. splitting them apart (Y stage + X/Z stage vs. X/Y/Z stage). Namely - the primary axis will be moving considerably more mass. However, the Watson reprap attempts to leverage the advantages as much as possible by rearranging the common components and stiffening the application of & reaction to force. Namely - the Z stage is the least demanding in terms of acceleration and so it is run by a single motor with a belt/pulley system. The Y stage (primary axis) in a gantry setup becomes the most demanding and so the 2nd motor is relocated to this stage to provide maximum acceleration. Intelligent firmware should also be utilized which uses proportional speed control depending on the Z position. For example - at low Z heights, low rotational inertia of the print head can allow higher acceleration while at maximum Z height, lower acceleration may be necessary. This should still net an increase in efficiency since for most parts low heights will equate to more build volume which decreases as the part grows in height (pyramid part geometry).