The shower should be at least four feet long with a glass door and, preferably, a seat. A good exhaust fan is a must. Adequate lighting, mirrors and storage space are also very important. Any good master bathroom should include all of these features. Once you've got the basics, you're ready to think about the luxuries.
Heat lamps are very affordable and add a huge comfort factor.
Towel warmers vary greatly in price but are also a generally affordable luxury. Heated floors will put a much larger dent in your budget but will take away the shock of cold floors against bare feet. If you decide to splurge on this item, take the full plunge and have the floor of the shower heated also. For the truly temperature sensitive, heated toilet seats are also available. Bathroom mirror defoggers are another nifty item.