London:
There's plenty to do, but once you've done it, the place itself is awful. It's overcrowded, overpriced, full of crime, and has to be one of the ugliest European capital cities outside the former Soviet bloc. It's architecture is a jarring mishmash of classical antiquity, visually oppressive 1970s brutalism, and neo-futurism, and it's culture is no less disjointed: most of the people you run into are a mixture of tourists, broken-English migrants, and bored commuters, and the prevailing atmosphere throughout the city is completely soulless. The best part about visiting London is the train ride home.
Paris:
The biggest disappointment about Paris has to be the way that the reality is so different from the image people typically have of it. Before I went, my perception of Paris was that it was a beautiful, glamorous, and sophisticated city, but when I arrived, I was greeted instead by litter, homelessness, warning signs about pickpockets, gypsies begging, and gangs of African migrants harassing people to buy their cheap trinkets. It is a beautiful city, and like London, there is still a lot to do, but the aforementioned drawbacks are a huge spoiler, and like London, it is also hugely overpriced.