Every style of mass build home and every era eventually has some feature that ultimately makes it look outdated. By mass build I mean the 'whole neighborhood' style house ... as distinct from super high end homes that are less prevalent and may or may not have the typical emblematic features.
Today's mass build style is obviously the arts and crafts / craftsman. Any care to offer a guess as to what feature / design element is most likely to seem "so 2014" in 15 years? I could see the symetrical dual triangle roofs becoming a hallmark of the mass appeal craftsman:
http://www.redfin.com/VA/Arlington/3400-N-George-Mason-Dr-22207/home/11226639
I also am not so confident that the open floor plan kitchen inside the living room will be a lasting trend. That's more of a longshot but I have heard more people asking for a little separation (not entirely different rooms, but some offset and/or half walls).
Other guesses?