I think we just had this discussion...and I basically agree with Delta and Aeric...
D&D is not realistic: It is not a realistic depiction of any era. And like most stories and games, it does not wallow in the misreable details of day to day life. But again, it doesn't do this for any era.
D&D is not modern: Healing someone with techniques developed scientifically is modern. Healing them because your god gives you the power to is not. D&D does not have fast cars, automatic weapons, or the other tropes of moder action adventure. It does not have charecters going to the dentist, waiting at the airport, getting colds, working in cubicles, or any of those details of modern day life. And it gets worse...
D&D charecters kill sentient beings and take their stuff: What most PCs do is not very nice. Its not specifically medieval, and the game helps rationalize it, but the point is for charecters to behave in a way they really couldn't get away with in their own lives, or for that matter in most civilized places, at least outside of particulalrly violent era.
D&D is closer to medieval then anything else: No printing press, no guns, rigid class structure (don't forget those commoners, their part of the rules now), late medieval weapons and armor inconsistent with an ancient world setting...And in many campaing settings rule by kings and nobles, urban guilds, water and wind power, castles and walled cities, and of course the acurate belief in a whole range of creatures and other superstitions, no matter how warped for our gaming pleasure, which have medieval roots.