If not well maintained, the porous material is prone to staining and chipping.
Wax concrete counters monthly to protect against stains, and use a food-safe sealer, like Cheng's ($27 for 500 milliliters; concreteexchange.com
), once a year. To erase discoloration, sand with a fine-grit diamond polishing block (about $10 at specialty hardware stores), then reseal.
Pass on urethane-based sealers; they can weaken if moisture penetrates. And don't set hot pots down on concrete; the heat can cause flaking.