Mary Sues, definitely.
Usually the kinds of ponies or beings you would never expect to see in the show. While they're fun to draw, they're not really good for roleplaying, fanfictions, and the like. Though, crossover fanfics are a different story.
Related to a main character, especially if it's one of the mane six. Things like sister, brother, cousin, uncle, aunt, whatever.
If you think alicorns are generalized, look at recolors, especially Rainbow Dash recolors. Though, people hate recolors for a more logical reason - it's just so boring, and the Rainbow Dash recolors seem to be a lot more common than the other recolors. I want to see what you can come up with, not your color palette of someone from the show!
Now, here's something that I don't really think makes an OC really poor, but it's something that I kind of dislike... Unreasonable cutie marks/talents.
Example: I understand that video games are canon in MLP, but the only video games we've seen so far in the show have been arcade games. Why is your pony's cutie mark an Xbox controller or an Xbox 360 logo?