Trying To Attain 100% Completion On Red Dead Redemption, What Am I Missing?

You don't mention anything about the bounty locations (20 of them, scattered between New Austin, Nuevo Paraiso, and West Elizabeth), outfits (9 specific ones you need), the four main challenges, or winning each of the 6 minigames. You can check for specifics on each of them in the source link. You can also check out the Rockstar Social Club if you've been playing the game online, as that keeps a record of your progress towards the 100% and tells you what is missing. Just to forewarn you though, the site does seem to glitch a bit, as mine is stuck telling me I am missing a piece of clothing I definitely have.