Interesting how the management sheep at Heathrow have really messed up their plan for incrementally transferring to T5, whereas Eurostar got the complete overnight move across London pretty much perfect. It seems that the biggest early problem with St Pancras was contention for the sole coffee port.
In digging for the BBC's actual "it's open" story (which doesn't seem to exist - maybe good news is not news) I found this:
allowing trains on the British side of the Channel to run as fast as those on the French side, at a maximum of 300km/h (186mph). But speeds in tunnels will be limited to 270km/h (168mph) for safety reasons.
Someone I know who works for Network Rail said over a year ago that CTRL had goofed on their air pressure calculations, and the tunnels are bored to too small a diameter to allow 186mph running. Whether that legitimately counts as "safety reasons" I don't know. I have no independent evidence or citable sources to back this assertion up, but it's consistent with other cock ups he tells me about.
