VGFacts

There is a theory that the main character, John Marston, has a blind left-eye.

http://reddead.wikia.com/wiki/John_Marston#Trivia
There is some evidence supporting the theory such as after using Dead-Eye, he will sometimes comment "I think like I got my eye back."

His Deadly-Assassin outfit includes an eye-patch that goes over his left-eye.
http://reddead.wikia.com/wiki/Deadly_Assassin_Outfit


During cutscenes, John seems to rotate his head further to see more clearly to his left side.
http://youtu.be/H6GZCD-te3I?t=4m33s
http://youtu.be/H6GZCD-te3I?t=6m20s
http://youtu.be/H6GZCD-te3I?t=14m38s
http://youtu.be/H6GZCD-te3I?t=24m40s
http://youtu.be/H6GZCD-te3I?t=35m3s
http://youtu.be/H6GZCD-te3I?t=42m16s
http://youtu.be/H6GZCD-te3I?t=58m26s
http://youtu.be/H6GZCD-te3I?t=1h2m3s

After Dutch http://youtu.be/IMwg-6vTyW8?t=5m6s

The Undead version of John is missing his left eye.

http://reddead.wikia.com/wiki/Undead_Cowboy_Outfit


There's also another interesting bit hinting at this, which is a comparison between John Marston and Johnny Klebitz; Both share similar names, both rode in gangs that left them both for dead and come from rough backgrounds, both have antagonists named Bill/Billy, both had a girlfriend/wife that was considered "everymans", both are forced to work with the government to save themselves and those they care about, and it is implied Johnny has a weakened/blind eye. ("Oh. I should use my good eye.")
http://gta.wikia.com/Johnny_Klebitz#Trivia

Interesting - It's a well thought out theory. Would love to know if this was intentional.

John Marston and his wife Abigail Marston share the same names as the second President of the United States and his wife, John Adams and Abigail Adams. Both families also had a son named John as well ("Jack" is a diminutive name for John) and both had a daughter die very early in their life.

http://reddead.wikia.com/wiki/John_Marston
http://reddead.wikia.com/wiki/Abigail_Marston
http://en.wikipedia.org/wiki/John_Adams
http://en.wikipedia.org/wiki/Abigail_Adams