One difference with online dating is that people tend to objective and quantify the other person more easily. In person, there tends to be fewer traits known before getting to know the person. Online, you have a dossier that causes more candidates to be rejected without a chance to get to know then.
In some ways this is great because you don't waste your time, or their's, if there's a serious incompatibility. But I think it can also be used negatively when people are too picky. Sure, there are people who are too picky in real life, but those snap judgements are based just on what you can see. The absence of other information can make someone go, "Ok, I'll give them a shot even if they're average looking because maybe they're interesting/mysterious/etc".