From the messages you've posted, D is an extremely long symbolic expression. While you might be able to read it, I'm skeptical that you'll find it useful or informative. A quick Google search says that a rule of thumb is that one 8.5" by 11" page of text is about 3000 characters, meaning that if you printed your symbolic expression it would span at least 8 pages or so (if I remember the threshold for the "Text exceeds maximum line length for Command Window display" message correctly.)
Now if your symbolic expression is that long, it's likely to be at least somewhat complicated. I'm not sure if your simplify command hung or if it is just taking a very long time to try to simplify that complicated expression.
Some suggestions for ways to do some simplification of D: use subs to substitute in numeric values for some or all of the symbolic variables, use vpa to approximate the symbolic form of a numeric expression as a number with perhaps fewer digits, extract common subexpressions using subexpr, convert it to a char and write that char to a file on disk as madhan ravi suggested, or maybe convert it into a MATLAB function file using matlabFunction with optimization enabled.