"The design of a bathroom must account for the use of both hot and cold water, in significant quantities, for cleaning the human body. The water is also used for solid and liquid human waste to a sewer or septic tank. Water may be splashed on the walls and floor and hot humid air may cause condensation on cold surfaces.
From a decorating point of view the bathroom presents a challenge."
(pic from Cowboy Antiques)
(To all Good Quotes)
Since no one is starting this, I'll throw out a line. I think the biggest challenge in designing a bathroom is not moisture or (ahem) effluent, but storage. Integrating enough storage without crowding the space or making it look like a bath showroom -- cabinets and shelves galore -- is difficult. I use freestanding furniture pieces instead of built-ins because they are often better quality than ready-made vanities, they cost less than custom cabinetry (and can be customized with new hardware or paint), and they can be changed as one's needs change. Cabinetry is often the first thing to look tatty in a bath. Also, I like to see under cabinets. I don't like any furniture that runs directly to the floor. Raising the furniture up gives the feeling of more space...in any room.