1)Phishing, if your private keys and password are compromised your money is gone.
2)Regulations, although governments would find it difficult, if not impossible, to regulate BitCoin. They can regulate the exchange of BitCoin into everyday fiat currencies. We seen this with the likes of e-gold when exchangers were cracked down on due to the ease of laundering money in the system.
3)Value, if people have no faith in the value of BitCoins then the system will not work. Right not there's a lot of commerce with BitCoin but it seems that it's only digital products or services that are accepting it. Once sellers of physical products start accepting BitCoin then it might become are more common currency.
4)Drugs, CP and money laundering. Although I don't believe in the scaremongering tactics used by politicians in order to regulate the likes of the internet, this system is going to be vulnerable to the similar attacks. We've already seen narcotics marketplaces open up that are now accepting BitCoin by operating through the TOR network. Soon enough this will receive a lot of unwanted attention.