A Mayan long count cycle ends on 21 Dec 2012. Much has been said about this, maybe even a little hysteria. No one gets crazy when they think about our calendar ending each year on Dec 31, because we know that a new year begins on the next day, 1 Jan.
One difference is that there is a grand astrological occurrence that occurs with the end of the current long count cycle and beginning of the next. Since celestial bodies affect each other in ways that remain mysterious to me, this alignment will affect us. Here is one explanation of those effects:
Whether you notice any difference or not, Happy New Mayan Long Count Cycle, which occurs on the winter solstice! It also occurs once in about 5,126 years.