One important question to ask is, at what point did these creatures begin their underground lifestyle? Did they develop tools, lighting, and social structure beforehand, or have they been burrowing since before they were sapient? This may actually result in very different forms, depending on how well they were able to modify their environment to account for their lifestyle.
If they were a group of humans that moved their civilization underground, they might actually wind up shaped not especially different from your typical fantasy dwarf. Small size would allow them to save on time and energy by making smaller tunnels, and a stocky build would make them proportionally stronger, which is helpful for digging through rock. If they had artificial lighting already, they might develop better night vision and more sensitive eyesight to allow them to see better with the sparse lighting they have available. They probably would not experience the more extreme types of evolution, since they would be able to modify their environment through technology. The way their culture evolves, however, would be interesting to think about.
If they started out as underground creatures, however, they might wind up with very, very different shapes. If they lived deep underground, with no access to any light at all, they would probably lose their eyesight altogether - unless they went the opposite way, and developed bio-luminescence instead, possibly by forming a symbiotic relationship with an existing bioluminescent organism. They would probably not remain bipedal for very long, as burrowing animals typically use either their front legs or teeth to dig, and without tools the benefit of standing upright would quickly be lost in a subterranean environment. As is the case with ferrets, weasels, and burrowing lizards, they would probably end up with long, flexible bodies and stubby limbs adapted to burrowing rather than grasping. It is difficult to imagine how such creatures could develop civilization in the same manner as humans, although they may discover fire through sulfurs and pyrites. A race of intelligent mole-like creatures who developed chemistry before they learned to make tools would be very interesting to explore!
Some adaptations that could apply in either case could include thicker skulls (which could protect them from falling rocks), slower metabolism (to account for lower access to food and oxygen), and thicker, more durable fingers. Their ears may experience some very interesting adaptations; on the one hand large ears are vulnerable to damage, on the other hand good hearing is helpful in low-light environments, and they may well develop some kind of sonar ability. They may also lose color vision, which is unreliable without a great deal of light, and would probably lose most of their skin coloration within a few generations, which would both help them perceive each other better and would be less of a problem in a world without sun. If they used fire, an ability to detect explosive gases instinctively, whether through smell or sound, would be very helpful. Hair is a toss-up; on the one hand body hair can become full of dirt and dust which may make clean living in underground areas a problem, on the other hand thick body hair can offer an extra layer of protection from scrapes and bruises, and long whiskers can even double up as a sensory organ as it does in the case of cats and rodents.