My only real gripe is the need to waste half a day trekking across London to the expo. I can see why they do it, as it must bring in a welcome amount of cash for the organisers, but it adds a lot of stress just when you don't need it. I'd rather be putting my feet up and watching Chariots of Fire.
As for the T-shirt, my wife wears it as a night dress, so it isn't too bad. Maybe it's designed to fit the guy in the rhino suit? She also loves the Jelly Snakes in the goodie bag, so I really hope they keep those.
Apart from that, the marshals are really friendly, especially at the start and finish lines, the support is unbeatable, and the atmosphere is fantastic.