Risk management is probably the single most important thing to understand in trading. Unfortunately, it's something that lots of people learn late, if ever.
Folks get caught up in the romantic notion of betting it all and winning big, but end up losers. Meanwhile, the consistent winners they aspire to be are exposing perhaps 0.2% of their roll at a time.
Though, one thing I think is a bit unique to trading is prevalence of folks who preach without practicing. Just about anyone with a brokerage account can rattle off the same short list of critical do's and don'ts, but very few actually follow them.