Self selecting and perpetually reinforcing population of young people living a certain life style you have to adopt to fit in... Or I could stop beating about the bush and flat out say it's just too cut-throat, crime-ridden and debauched for my liking. There's also pollution, traffic, over-population, poor living conditions (even adjusted for the increased salary) and crime (worth mentioning twice). At that point, it matters little how vibrant, exciting and full of opportunities it might be.
Isn't this the same for most big cities though? I'm not a massive fan of London, it's just where I have to work, but I think you'd find the same amount of crime and over-priced flats in places like NY.
I have a love-hate relationship with London. It's crowded, dirty, unfriendly, expensive, the weather is bad (like the rest of the UK) and the taxes are high (unless you are from NW Europe). That said, I keep coming back here. It's the best place for work experience, and full of opportunity. It's hard work, nothing is easy here, but if you put in the effort you get the reward.
There were two replies at the time I started writing this. I actually find it funny how different a city can be for different people. Personally, London quickly become my favourite city (New York City used to be the previous favourite one). Unlike one of the other replies, I find London to be much more safe than usual for a big European city, I never felt this safe in a big city before. I also never had such a good overall quality of life before coming to live here. You can be whoever you want to be on any given day since there's so many people here noone will notice you changed if you really want to try something new. Given the salary you can get over here, I even find London cheaper than most other big cities (for example, supermarket food seems to cost around the same as the cheapest big cities in Europe yet my salary is way bigger over here). Obviously some things are crazy expensive like public transports and taxis.
