Designing data structures "on the fly". it's the same with programming.
Every programmer thinks they can write a whole application from the keyboard and 99.9% of them can't.
If you do not start with a reasonably well formulated plan, the end result will be something like early versions of oSCommerce!
A mish mash of bits and pieces, loosely cobbled together with hacks and kludges.
When you come across a "feature" that "would be nice", add it to a "todo" list and build it in later as a "plugin" or "module".
And Dreamweaver cannot design databases.