Yard boots must be one thing above all: comfortable! After all, they should support you in your daily tasks around the stable, which involve being on your feet a lot (and therefore in your yard boots).
Non-slip soles, firm hold, being easy to put on and take off - all these are important criteria when choosing a goodyard boot. A good foot bed and firm support in the ankle area are also important points, but it's also important that you like the look of the boots, so that you always enjoy wearing them.
For winter, consider a yard boot with a lining to keep your feet warm. A water-repellent or waterproof upper material is also important for mucking out and other barn chores.