Most of us visit the Olympic Games as a spectacular and cannot fell the unstop or high pressure intensiveness that the organizations have every second. We can only see the outcomes of their hard work. To be a host city of Olympic Games is serious adventure, because you prepare the whole for something big.
Looking from financial point of view, the city must spend huge amount of money which will come from the people taxes etc. So far, the London municipality estimated over 20 billion pounds as a total investment, which is a number passing the Beijing Games.
On second point, you have to deal with security measures and safety of the people. London has been a target for terroristic groups from a long-time and this is not giving a peace for the police groups to settle extremely high security measures. Also, don't forget the contingent of VIP guest which will extra back up and "hot line" to the hotel and Stadiums. What about the local people in London, who will feel all these on their backbones.
Looking from the good point, this is giving London popularity and huge money investment which will pay back in the future. Landlords in London will have the opportunity to rent out their properties in order people to have the ability to book a flat for Olympics 2012. This is one of the biggest opportunity to have an Olympic city host, because the travel market will revenue a lot from the accommodations London 2012.