I love doing this. Old floors make great countertops and cabinetry too! And sometimes you'll have wonderful colors appear subtlely in the grain from old stains by sanding them down.
If you use boards for cabinet doors, I recommend using a strong back on the backside to keep it stable.
Wood kitchen counters hold up surprisingly well. You can use a nontoxic clear finish or simply oil them. The finish would depend on what kind of wood floor you have. Of course, the harder the better.