Okay so far I've diagnosed several mental illnesses/disorders and possible drug addictions in these characters:
Fluttershy, Schizophrenia (Notice the way she talks to animals and acts if they're talking back. That isn't normal.) and PTSD. (Watch the episode where she's trying to help in that hurricane thing and has a flashback to the day she had stage fright and the other kids laughed at her.)
Rainbow Dash. Not really sure. To be honest she's one the few main characters besides Applejack and Spike to appear completely sane.
Rarity. Blatant Narcissist.
Applejack. Mentioned before I actually haven't noticed anything odd about her.
Twilight Sparkle. Obsessive-Compulsive Disorder. I don't need to explain. Anyone of you who knows what OCD is will agree with me immediately.
And of course, Pinkie Pie. I've discussed this before. She's on some type of drug or has some kind of hyperactive/manic personality disorder.