When have you ever read, or even heard of, a book/movie/graphic novel/whatever in your favorite genre that had a main character who was mentally disabled?
My challenge to you is to write that story, or at least the premise of one.
There's quite a few disabilities to choose from:
A character with Autism has distorted or overloaded senses, and will easily get overwhelmed by new textures, changing appearances, and facial expressions. They tend to like repetition, organization, and simplicity. What would happen if Eragon from the "Inheritance Cycle" were Autistic?
This is actually a variant on the Autism spectrum. Asperger's Syndrome is often called a milder form of Autism. People with each will share many symptoms like repetitive motor movement, having issues with normal speech, and obsessive interests like collecting bottle caps. However, a character with Asperger's likely seems more "normal" to the average observer, and wants to socially interact (he or she will just do so somewhat awkwardly). Imagine Harry Potter with Asperger's Syndrome. Oh, wait, he already has communication issues.
Contrary to popular belief, it is very rare for a person with Turrette's to compulsively spew swear words all the time. If your character has Turrette's, she or he will have a tic. A tic is a motor movement or verbal sound that someone repeats unconsciously, and may do so more often under stress or when concentrating. It can be repressed at will, but usually not for long. A motor tic can be a twitch, a nod, a wink, touching other, stretching, or even hopping. A verbal tic may be a grunt, saying the that last syllable of a sentence twice, or word or even a phrase. Hm, sounds like all Moogles from Final Fantasy and Kingdom Hearts have a verbal tic, kupo.
OCD people are not actually all obsessed with having things neat and orderly. They could have plenty of rituals, obsessive thoughts, and compulsive behaviors that have nothing to do with cleanliness. As long as the action, or series of actions (ritual) include obsessive thoughts and an incapability to do without them, it's OCD. A likely reason for a character to develop OCD is that he or she developed a (sometimes) irrational solution to a fear that caused him or her trauma. For instance, a child might check the window over and over for lightning (even on a dry afternoon) because his curtains were closed on the one day that lightning struck a tree that fell down on his house. If he'd only seen it coming, his ribs might not have cracked, right? Maybe, maybe not. But all the same, he can't stop checking the windows every ten seconds, every day. I wouldn't be surprised if Charlie the Unicorn developed OCD...
Of course, those are merely a few mental disorders out there. I suggest you do your own research to find what disorder might fit your character best!