Very interesting set of questions. I know that HasTak starts out their designs flat-surfaced for their prototypes and such at first, and then in later prototypes and versions of the model they add more detailing and the colors. Though, I'm not an expert on how HasTak do it, but I know from experience how I prefer to do it, and what I look for in figures.
Detailing on a specific figure being designed really depends on what theme/role/alt mode the character might have. If it's a character with a tank sort of mode for example, lots of hatches and storage areas are things I generally add. And things like planes, jets, and other flying vehicles, I generally prefer smoother designs of the armor, as well as more streamlined details over bulky ones.
Now as far as details that fit the character, that is an interesting observation for sure. I find that characters who are meant to be tougher or scarier generally benefit from more lines and technical detail. Things such as longer necks with more wiry details work well on weaker or creepier characters, while more tubes and pistons look very nice on bulkier and more tanky characters. And with smaller characters, I find that less is better when it comes to detail. In my mind, larger characters look even larger if they have much more panel and technical detail than smaller characters.
Those are just a few of my thoughts. Hope it was at least a little interesting.