As an EU citizen you get interrogated why you're visiting the U.S. When I was in SF for WWDC, it was easy, "I'm here for Apple's WWDC". Any other time it's been a 15 minute ordeal of explaining what computer software is exactly and how my international contractor relationship works and why I'm not going to escape into the forests and become a day laborer. Once they even asked for a bank statement. In no other countries I've traveled to (20+) have I had to explain why I'm there aside from checking a box on a paper form.