One of the filter options is to show unused layers and linetypes. Also investigate Layer and Spatial Indexes to reduce the amount of memory required to display a drawing. If a dimension is placed without associativity, a value that contains no fractions will be placed a certain distance from the dimension line. If the value is placed with the old style AutoCAD fractions, it may be placed at a different distance. If it is placed as a stacked fraction, it may be placed at a different distance resulting in dimensions that look like they are riding a wave.
Middle center is the half way point between the top and bottom of text.
A user not well versed will have no idea what happened or how to correct this. This tells the program to center the numbers. It may be desired to show new or less experienced users this method so that they may understand what happened. A new, really needed, feature in R14 is the ability to do transparent zooms and pans meaning that while you are inside a command you can zoom out, go to another location, then zoom back in to resume the command such as drawing a line from one point to another. It really saves time and aggravation.