UK border security. On Eurostar journeys all border checks are done at the departure station, and only Paris, Brussels and Lille have the facilities for that. That said, it should be coming this year.
Interesting. I assumed there was one! I regularly take the train between Berlin and Amsterdam and it's awesome. door-to-door it's actually only slightly longer than a flight, but so much more comfortable. I usually get either a ton of work done, or I relax with a movie in the restaurant area. Flying, on the other hand, is about 50% just waiting in line and/or being uncomfortable in some way.