Concrete flooring works for bathrooms for the same reasons that ceramic tile works for bathrooms; it's easy to clean and it can't be damaged by water. Not surprisingly, the same drawbacks apply, too. Concrete can be cold, and it can be slippery if it has the wrong type of finish or surface treatment. It's also very hard, but this is a less significant drawback in the bathroom than in the living spaces of a home.
Concrete Floors and Water
No amount of water can actually damage a concrete floor. After all, they make dams and underwater bridge supports out of concrete. This means you never have to worry about wet feet, damp towels, shower spray, or spills from the sink or tub. What you do need to think about is traction. Interior concrete floors can be finished until they have a very smooth, almost glass-like surface. In wet areas, like bathrooms and kitchens, you don't want to go this far. A polished concrete finish can be slick when wet, especially when it's treated with a surface sealer.
Slipperiness and the rock-hardness of concrete is a bad combination. If you're having a new concrete slab or concrete overlay poured for a bathroom floor, make sure the finishers leave it with a little traction for safety. Do the same if you're grinding and polishing an old concrete slab to use as a finish floor.
Concrete Can Stain
Concrete is a naturally porous material that is highly susceptible to staining from spilled liquids. This isn't as much of a concern in the bathroom as in a kitchen or dining area, but it's worth taking precautions to prevent problems with staining and discoloration. The best solution is to seal the concrete periodically with a film-forming sealer. Some sealers add a bit of traction to the floor, which is a good option if the floor is too smooth for safety in a bathroom.
The Cold Question
Concrete slabs tend to be colder than floors over wood framing because slabs typically sit directly on the ground, which stays at a relatively cool temperature year-round. Concrete also has a high thermal mass. This means that once it's cold it stays cold, and once it's warm it stays warm; in other words, it changes temperature slowly. If you're heating a bathroom with conventional forced-air heat, you're simply blowing warm air over the top of the floor and throughout the room, so you can see why this would have little warming effect on a slab that's being chilled by the ground 24 hours a day.
On the other hand, if you're pouring a new concrete slab, you can pour the slab over insulation (as a thermal barrier over the ground) and lay electric wires or hot-water tubing inside the slab to heat floor itself and, in turn, heat the room. Due to its thermal mass, concrete is an excellent material for in-floor heating.
Care and Cleaning
Concrete scores very highly in the cleaning and maintenance category. Aside from periodic sealing of the surface to prevent stains, concrete is effectively maintenance-free. For everyday cleaning, you can sweep, vacuum, dry-mop, or wet-mop as much or as little as you like. Concrete is not like hardwood or carpet, which wear much faster without regular cleaning. Just be careful with cleaning agents, as some cleaners may react chemically with the concrete and cause discoloration.