These are the main reasons, why I believe Online Dating isn't a good option. And from what I see in various conversations in different websites (including GAG), Online Dating is proven to be unsuccessful.
(In Alphabetical Order)
1) Disappointment When You Meet Each Other In Real Life
It's pretty common to happen. Online communication doesn't give you an 100% accurate impression about the other person. So, in other words you might have created an ideal image about the other person, and when you meet each other in real life, and talk for a while, you're going to experience disappointment.
Also there's a good chance for the other person to be fake. It's not hard to pretend to be someone else, when you talk online.
2) It's Usually An Option For Unconfident and Shy People
I believe it's better to try to build up your confidence and take your chances by hitting on someone outdoors. By choosing to date online, it's more or less the "easy way out". Everyone is more confident online, and by choosing that as a dating option, it won't help you build up your confidence.
3) More Chances To Cheat Or Being Cheat
Actually I'm one of those people, who don't mind cheating so much. Yet I'm aware that most people mind about it. Well anyway, this is one of the biggest flaws when it comes in online dating. You don't see each other since you are miles away (usually) from each other, so there are no "I saw you with him/her". In other words, both are free to do whatever they want, without having to watch their back.
4) Slow Progress
You won't meet each other from one day to another obviously. It takes time and many Private Message sessions to get to know each other better, and finally meet each other in person. #4 is linked to #1 actually. You will have a better impression if you knew the other person in real life from the start, than knowing him/her online first.
5) You Spend More Hours On Your Computer (Indoors)
You have to use a computer in order to talk to that person, obviously. Which means you have to stay in home and use your computer more. It will eventually distract you from other activities, but any type of relationship can be distracting actually.