I know there was just an official blog about this, but I was typing a response to this blog and got a bit carried away. Rather than leave it as a ginormous comment, I figured I'd just post the long version as my own blog and cut down the comment version.

To give you a bit of context, the blog in question essentially asks why red-and-black and alicorn OC's are frowned upon. The author claims that many readers (particularly of the train wreckers crew) judge these OC's based solely on their appearance instead of the actual writing. Many other writers have replied by saying that it's not the color scheme, but the lack of character development. In that spirit, I decided to write my thoughts on what makes a good OC, as well as how I've avoided the red-and-black plague.

I've created about 30 OC ponies in the few weeks I've been writing MLP fanfics, with varying degrees of complexity, and the hardest part by far is coming up with a physical appearance. I'll admit, it isn't very easy to come up with a unique color scheme and style after a while. However, I find building the character before I build his/her appearance helps a lot. Before I picked the color schemes and mane styles for almost all of my OC's, I sat down with a piece of paper and wrote down all the things that made them... well, them. Then I pick a color scheme, mane and tail style, and other characteristics that suits that character for who they are, not what necessarily looks awesome.

Here are a few questions I like to ask myself when coming up with a new OC.

1) What is his personality? Why?
2) What does he want? Why? Why can't he have it?
3) What are his strenghts? Why?
4) What are his weaknesses? Why?
5) Who are the most important characters to him? Why?
6) What is his role in society? Why? In the case of ponies, what is his special talent and cutie mark? How did he get these?
7) In the case of ponies, what race is he? Why?
8) What are his physical characteristics (size, build, coors, mane/tail style)? Why?

Notice the kinds of questions I ask, and also the order in which I ask them. The most important thing about a character is who he is as a person (pony?), not what he looks like and what he does for a living. What are his goals and ambitions? What are his inhibitions and limitations? Who matters to him? And, above all, why? Unless you can explain why, your character lacks motivation, and he becomes unbelievable (no, I don't mean that in the good way).

Only after I've build the character do I worry about his physical appearance, mostly because knowing who he is will tell me what he should be. Let me use Scrollkeeper as an example. I won't say he's a brilliant OC, but this process helped me avoid many Gary Stu pitfalls.

Circumstances and history:
-Librarian
-Pegasus
-Older
-Lives in a library with his nephew, Cleansweep
-Lived in Canterlot before it was destroyed
-Specializes in Equestrian history and lore
-Cutie mark is a scroll

Personality and attributes:
-Logical
-Realist
-Strict
-Honest
-Thorough
-Reserved
-Polite
-Patient

After I did all that, I went to a generator (yeah, I'm no good at art) and found an appearance and scheme that fit him. He's a pegasus, so obviously he had wings - normal wings, mind, not the weird robotic ones (though I have seen those used well). He's older, so I gave him a graying mane and tail, both of which had simple styles. Based on his age, life experiences, and personality, I gave him a slight frown and narrower eyes. Nothing really suggested a specific coat color, so I picked a subdued orange. No stripes, no special marks. Physically, there's nothing really remarkable about him.

So, why is he interesting? Because he has a personality that conflicts with others. His nephew, Cleansweep, is an energetic and somewhat ill-mannered colt. How does that match up with Scrollkeeper's strict politeness? How does his logical realism clash with Color Wheel's outspoken optimism? He can even have disagreements with another strict pony, Galeforce, because the latter's zeal and quick-to-judge attitude conflicts with Scrollkeeper's patience and desire to find the truth before passing judgment. He can even be bent slightly out of character when faced with Gavel Law's blatant dishonesty and moral corruption because he has values and priorities.

This is why it's so important to build a character, not something that looks cool and can do cool things (hence they're called original characters, not original designs). Try to find a balance of positive and negative attributes, especially ones that can count as both. For example, Scrollkeeper's reserved, polite disposition and dedication to get all the facts first help him make better decisions, but they almost got one of his closest friends in trouble because he failed to speak up when he thought something was wrong. We don't hate red and black OC's because of their colors. We don't hate alicorns because they're alicorns. In fact, I don't really hate them at all. What I dislike about them is that almost always, they're a sign that the writer put more thought into the design than the character, and no matter how cool and powerful your OC is, if you miss the character part, you're in big trouble.