Most people will not drive a great distance for a class over time, so a class that is close will often be more attended. Similarly the classes must be at a time that is convenient for you to attend and must fit reasonably within your budget.
Another big consideration is the type of yoga you are interested in. Hatha yoga is the form we typically think of when we talk about yoga. Vinyasa yoga connects one breath to one movement. Bikram yoga is done in a warm (hot) studio following a pattern of 26 poses. Power yoga is a more athletic form of yoga. These are a few of the most common types of yoga.
Also the training, background and personality of the instructor will make a big difference in your yoga experience. Some yoga professionals study yoga very deeply and have a deep knowledge of all of facets of yoga. Other instructors come from various fitness backgrounds and will approach the study and practice of yoga accordingly.
One final factor to consider is how much of the spirituality of yoga you are comfortable with. Some classes and instructors have a distinct lean to the more spiritual practice of yoga, while others lean more toward the physical movements (or asanas) of yoga. You must be comfortable with the amount of spirituality in the class or you will not continue to attend the class.
As with any other class it is a wonderful idea to take a class or two with the instructor or facility before you commit to that class or facility.