Remember when the bathroom was a utilitarian sort of room? You went in there to use the toilet, get clean, take medicine and all that good stuff. At some point along the way -- and maybe this is Calgon's fault -- we started to think of bathrooms as a potential haven from the stresses of the day. Can't afford to visit a spa? You can have a spa experience in your own bathroom.
Usually the master bathroom gets the royal treatment in these cases, but once again, those things that were once luxuries are expected. No more fighting with a partner over the sink and mirror when getting ready to leave the house; people just expect to have two sinks and plenty of mirror space and storage. It's also normal to have separate "toilet rooms", a tiny room for just the commode to sort of close off that unpleasantness. (It makes sense that in Japanese culture, the toilet is always in a separate room from the shower and tub.)
So what's considered luxurious these days? While they've been de rigueur in many European bathrooms, only now are more people in the United States installing bidets. You can modify your toilet to perform a similar function, or -- in another nod to Japanese culture -- go for a high-tech toilet with a heated seat, water sprays and drying jets. High-end materials like soapstone and teak also make it feel more like a spa and less like just another room in your house. How about a really deep, really long soaking tub or a shower that's big enough for two or more, with multiple shower heads? It's not unheard of to enjoy music, TV or colored lights while you're getting clean and relaxing. Another trend is towards old-fashioned claw foot bathtubs.
Extend the experience by connecting the bathroom to a large walk-in closet, where you can sit on a chaise lounge and dress in comfort.