I am an online sub, and am still relatively new to the whole thing.
I have not had any bad experiences, but have had many difficulties with the whole concept of being an online sub.
I do have an online Master but, he is not a Dom in the sense that I need him to be. Without getting into the hows and whys of things, for now we offer each other what we need, and are satisfied with that.

I can go into a chat room and call anyone at that moment in time Master or Sir, but in retrospect, it is only for that moment, sometimes I get the need to play and that satiates whatever itch I am having.
To be able to be an online sub for anyone, I would need to get to know the other person on a much more personal level, and for me that takes awhile to do.

I have tried the Dom/sub relationship online, but cannot seem to connect with anyone.
I have had many Dom's task me first and offer to be my Master without ever getting to know me or understand how I click. Then again I may misconstrue myself and I could be just a very complicated person in general.

So to sum up my thoughts, to have or to be an online sub, you need the connection, and communication to achieve what you want, and as Sir_Russell stated you need to have limits and conditions discussed beforehand.

You had a negative experience, but you can learn from that and move on to more positive ones.