Now that I'm more focused with Russian, I would say that the "barrier" is also cultural because I'm very open, extrovert and I smile constantly and apparently Russians find it a bit weird that a stranger is smiling all the time so especially with Russian male partners, they look very serious and that makes me uncomfortable but again, culture!

My biggest barrier was with some specific nationalities and cultural shock.

When I was learning spanish some years ago, I found it extremely easy to find partners and they're very communiciative. In the first skype video we would feel very comfortable with each other, laugh and always find something to talk about. I used to have partners who I would spend 2 hours in a row with and sometimes they were not even learning my language so I didn't have to give anything back in return. 

But with some other nationalities, like Germans, Austrians, Swiss, that was actually impossible. I never found a single partner to stick for some time while with Spanish I used to have around 10. I would always target people who were learning my language, of course, but due to some cultural aspects, with German language was extremely difficult. I found them to be more reserved, so they were not as communicative as Hispanics and they were more difficult to be reached. As an example given, with Hispanics I would simply send them a message "Hey, are you available to chat now", if they weren't doing anything important at that moment, they would simply call me. With German speaking people, I would often hear " I can't now, I have to water the flowers " or " I have an exam in two months so I'm preparing from now and I can't talk with you until then ", or "I am reading a book now" they were always busy so with my experience was that Germans, Austrians and Swiss people were way more organised and they were strict with their everyday schedule, they wouldn't simply stop doing whatever they had planned in order to talk because that was out of their plan, So finding them available was really difficult. I only succeeded few times to have a skype conversation and that had to be planned days in advance. But I understand that this is simply a cultural aspect.

I agree absolutely with the last comment.

The greatest difficulty actually was the feeling that the language partners might get bored while I was searching for the correct words. As I am more an introvert type I also wondered whether I could keep up a conversation with strangers for more than one or two minutes. This goes almost only for speaking.

Writing is far easier as you can do it at your own pace. You can look up missing words and tune your sentences.

Either way I found that the first contact is the most difficult. After that things become better with every new contact and conversation.

It was difficult to find a person who was patient enough to correct all your mistakes and at the same time was engaged in conversations and interested enough while you're thinking on your preposition.

Speaking is even more complicated, as you feel intimidated and frustrated by your inability to find proper words in a short time and you feel like your partner is getting bored)

Last but not the least - you have to find a patient, interested, enthusiastic person... who at the same time shares your interests and way of thinking so you could actually practice, not just wandering around looking for something to discuss!) Well, that's a challenge, isn't it?)

For me, it was more about not having enough confidence in my ability to have a sustained conversation, so I started with teachers first. Only after I had already reached an upper intermediate level did I feel comfortable speaking with language partners.

For the question on what prevented me from getting started, I echo Daniel's sentiments about finding trustworthy sites. They're few and far between. In the case of Japanese, it's more because it's such a completely different language and the learning curve is pretty steep. So for that, I prefer working with a teacher because I'm not really conversational beyond the basics.