Something else you should point out is the "sparkle" phenomenon.

Using TOO MANY design elements on a character all at once turns them into an eyesore, and a lot of people use sparklecharacters as a hidden scam: To force artists who charge fixed prices to work more hours on a given character. (Those that charge hourly, like  owly, actually defeat that particular scam). I tend toward characters that have one or two TRULY distinguishing characteristics but that's it. You want just enough uniqueness to make the character memorable without turning into an eyesore!

That's a different issue, albeit a very annoying one. Thing is, the more complex the design is, the harder it is to make it nice if you don't know the basics. I've seen amazing sparkle/complex designs that were awesome to draw because they had a structure, a style or even a "rhythm" to them, while others are just... complex things layered without sense over more complex things.
That is why I have extra fees in place for characters that go way overboard with markings and details, plus the right to refuse commissions of said characters. I'm surely not going to make someone with a 4-winged rainbow-neon taur-dragon pay as little as someone with a basic white wolf would xD
In general, my rule of thumb for my own designs/adoptables/etc is "would I want to draw said character more than twice?". If yes, it's a good design in my book, otherwise I pass or fix it.