Gambling: When the odds are against you, it is in itself against reason. It feeds on our greed, hope, and curiosity. It can rob you of your money, time, and dignity.
Lack of education funding: Self explanatory, schools need money to teach kids. For the most part, the more the better.
Lack of encouragement for education in society: If no one is taught the importance of education, then many won't ever learn the importance of education.
Mainstream Media: Lets face it, most mainstream media is a huge waste of time, and brain space.
Psuedo-science: Wacky medicine, lies passed off as science, and half truths correlating to nonsense.
Recreational drugs and alcohol: Instant gratification, at the cost of risking a lifetime.
Religion: Stories widespread taken as facts with no evidence. Sometimes directly opposing reason, It's infectious ability to spread and to last is mostly due to people believing it is beneficial.
Subcultures: Might divert ones focus from reason, and replace it with a concern for trends, an irrational urge of attaining individuality by any means and strange ideologies.