Busy roads, traffic lights and some awful drivers - especially those on the school run I'm sorry to say. I'm actually a bit odd in that I live closer to the centre of London than my office and so commute in the wrong direction. I have some wonderful(!)stretches of 2 and 3 lanes on the A1 and the North Circular. Commuting to the outer suburbs of London is more dangerous than going right into the centre because the locals arent so used to sharing the roads with bikes.
One man critical mass! Never afraid to take the lane. Reluctant car driver when needs must. Tendency to over eat and drink - you can get away with it when you do 100 bike miles a week.