I don't usually like pooping on other people's OCs, but a few things are a no no for me when making original characters:
Unappealing Colour Design - Using colours that don't mesh well together, using harsh colours like deep red. MLP characters typically have colour palettes that work well together, and of lighter tone. A good rule of hoof is NEVER use black or red, and if you have to, use dark grey instead of black and a lighter tone of red... 0 Hue full saturation red looks awful.
Mary Sue - Really talented, No flaws, extraordinarily nice, everyone wants to be her boyfriend/girlfriend, etc... makes for a very dull character, although its kinda funny when done in jest.
Forced relation with main characters - She's Rainbow Dashes long lost sister! ... Not only is it kind of a groaner, but its just unbelievable. It's not much better when you put them in romantic relationships with the mane 6, either. And worst of all...
Recolor of an Existing Character - If you have to use an existing character's hair, at least try to add other stuff to help differentiate it.