Personally felt that the A380 was more comfortable then any other aircraft I've flown on, but thats an unscientific analysis.
As for the seatbelt sign, if it was unexpected then it's possible the sign wasn't on.
Sometimes however, on certain routes with constant turbulance at this time of year (such as this very route mentioned), the seatbelt sign can be on from pretty much departure until landing, which for a 10-12 hr flight does mean that people take the risk and visit the toilet.
Turbulence is environmental issue, that we just have to live with and accidents like this happen. Such as life!