One of the things I always liked, and found sort of interesting too, regarding customizing, was popping apart figures and seeing all the engineering that went into their design. They're amazingly simple anymore compared to a while ago, actually. In other ways they're more complex, but in many surprising ways they're much less complex than they were in say 1999/2000.
A lot of the simplification I attribute to cost cutting, not in only how they were constructed, but also the materials they were using. They definitely changed over the years.
Some of the designs of figures were fairly ingenious though... VOTC Chewbacca still rings as one of the more creatively designed figures ever IMO. Other figures were so high quality though, and not even recent ones. VTSC IG-88 comes to mind immediately, to me, as one of the most complex design and sculpted figures I think the line has ever seen, and it works pretty flawlessly. That figure's only real drawback is his blaster doesn't fit in the holster made for it. THat's probably more a flaw that came out during the manufacturing process though.
You also could see, popping almost every figure apart once (or more) over time, certain designer "trademarks" in a figure. You could tell the same guy had worked on this figure or that figure (most likely) by little things they did differently in its creation.