There is a lot of leeway to post dishonest information about yourself online. For example, people may lie about their age, height, or background. A lot of this happens in traditional dating too, but some people are more comfortable using deception in the context of the Internet because they feel more anonymous. It's also tougher to spot dishonesty online because a lot of information is exchanged by viewing people's profiles. In real life, you can look the person in the eye when you meet him for the first time, making it easier to assess his honesty.
Another problem with online dating is the deceptiveness of photography. People take pictures of themselves from all sorts of angles, and sometimes people can make themselves look more attractive than they actually are. People also often use outdated pictures that show them when they were much younger. If you're attracted to somebody primarily because of her looks, you might want to ask to see a few more pictures to make sure you're not being deceived.
Lack of Physical Presence
Dating involves a lot of information that isn't necessarily conveyed verbally. People express themselves with their posture and their facial expressions. People are also attracted or repelled for reasons that can't be conveyed without spending some actual time with a person. For example, you may find that a person's voice is especially attractive, or something about the way an individual smiles. Online, you don't have access to this information, so you might make assumptions about attractiveness that aren't true.
Another problem with online dating is that it can sometimes be too easy to dismiss someone. Sometimes when searching for romance online, it can seem as if there is an unlimited pool of people to choose from, and you might be compelled to wait for somebody who's absolutely perfect. As a result, you might dismiss perfectly good partners with minor flaws that wouldn't be a problem in a more traditional dating situation.