I think Online dating is similar in many ways to real life stuff, as people have said. If a bloke comes up to you in a bar or at a party and they start talking about sex, then you generally tell them to feck off. If they start undoing their zip, you call security.
If it's a reasonable, intelligent, amusing conversation though, you may be interested. The differences though are that people are more willing to mislead you because you don't have mutual friends to judge them for doing so, that you can't tell if there is physical attraction or something hugely offputting (or whether they're only 4'8) until you meet in person, and you're doing it alone, without some friends to protect you from the weird bloke or give you a sanity check on the guy you're chatting to.
Other than that, it's just the same, really. Arseholes, lovely people and lots of 'meh'
The Dating thread referred to earlier moves quickly because there are lots of people in similar situations, it's good to look at, at least, and offer advice if you have it.
Good luck