People register with dating websites to obtain e-mail addresses so they can send spam or viruses to unsuspecting users.
Your profile can attract someone who e-mails you constantly, even if you never responded to him. Or, after you see him a few times, he can start showing up uninvited to places where he knows you will be.
The Internet makes lying very easy. People can lie about anything they want in their profile: age, weight, income, marital status, if they have kids, their profession.
Online dating gives criminals another means to perpetrate violence; they use the setup to lure a date to an isolated location with the intent to harm or kill him or her.
Scammers who are just after money or information so they can commit identity theft string lonely hearts along with lies to get what they want out of them.
Lack of Courtesy
There are online daters who are inconsiderate. Because interactions are on the computer, you run the risk of situations like being stood up, getting rudely rejected for a date and being "dumped" after a couple dates with no explanations.