Outside of the centre, London is essentially a load of small towns without any countryside between them. Each has its own high street, some are nicer than others. Because of the proximity, you get very nice areas and rough areas very close to each other. Overall, the difference between haves and have-nots is starker in London than elsewhere in the country.
Londoners often cite the theatre, opera, museums, etc. as good things about London. However ask them how often they visit these places and them usually start umming and arghing.
Multiculturalism exists in plenty of other towns and cities too. However, this often takes the form of cultural silos that exist in parallel, with minimal intermixing. This starts in the classroom - a group of white kids in one corner, black in another, Asian in another.
When I lived in London, I liked the fact that public transport was good enough to mean that I didn't need to drive very often. Which was just as well, since driving there (e.g. on the North Circular trying to get out) is a total nightmare.
After any visit to London, you always take home the same 'souvenir' - black snot. The London air quality is a serious problem.
Anyway, enough of my rambling.