Also you want to make sure the ratio between different language speakers is well balanced. I can see the reason behind a lot a Brazilians and Hispanics wanting to learn English, it's the most important language after all. But are there as many English speakers (mostly Americans) who want to learn Spanish and Portuguese? What are the motivations for them to pick up a second language?
I agree with most of the points you made, however, as ccarpenterg just said, it's a sharedtalk for beautiful people, people who are introverted or not confident about their physical appearances might not be willing to video chat with strangers. In the first case you mentioned, were user willing to exchange skype names before or after they had the text chat? The reason I am asking is because text chat might make a very big difference in building the initial trust between users.
I am a college student who is currently doing my summer internship at a megacorp as a .NET MVC developer and I would not say the internship was what I was looking for. I am OK with the technologies they are using, it's just the boredom caused by endless waiting between each process that frustrates me. I got hired because I had spent a lot of my spare time working on my own RoR projects and my web development skills made me stand out. I also turned down another RoR startup internship as a result of better payment from the big company, which I regret a lot by now. Lesson learned: money is not the most important factor when it comes to job decision. The bright side of big corp job is that I have plenty of time to read hackernews and pick up technologies I want to learn, which gets me ready for the future startup environment. But nevertheless, I will never look back after this job.
It's funny that I came up with exactly the same idea this spring and was almost going to apply for YC funding. But I changed my mind after talking to a couple of my friends (international students who know more than one language), most of them said it's a good idea but they also felt it's not very comfortable for them to talk to strangers face to face (online). Another challenge facing such kind of site is how do you motivate strangers to actually teach each other languages while keeping their attention span long enough, say, 5 minutes. Without a good motivation system verbling will just end up being another video chatting site. I also couldn't find an easy way of generating profit besides selling ads and charging user for talking to professional language teachers. Maybe the site will build a reputation system so the best users can get paid talking to other premium account users, and site will take commission from each transaction. But in order to do that, you need to have a big user base, after all it's a chicken and egg problem.