I'm not a native speaker. However, I have tried a lot during last 10 years to learn English at a high level of proficiency and to become fluent in conversation.
However, when I talk to some of my friends in US over Skype (found via my profession), they tell me that I talk like a foreigner. But they don't know why is that.
What is/are the reasons that can make a not-native English speaker sound like a foreigner? Is it about pronunciation, or about syntax, or is it maybe just a false assumption coming from a psychological background due to the fact that the other party knows that you are not an English man/woman.
Update: I asked this question specifically about English, since English is the most widespread language on the earth and tens of different dialects and millions of different idiolects seem to alleviate the problem of "sounding-foreign".