Positives:
I LOVE the diversity of it. I come from a small village in the countryside where everyone is white (literally, not a single person in my school was not white) and English - which is not inherently bad, but from my experience, can become a breeding ground for racism, xenophobia, and insular thinking (all of which I detest). I also love how many great places there are there, in terms of businesses, museums, coffee shops, attractions, etc. It has a really exciting "buzz" about it. There's nothing more magical than London at Christmas!
Negatives:
It feels dirty, especially compared to some other European capitals (e.g. Vienna, Stockholm). People can feel quite cold, quite busy, quite stressed. It is absurdly expensive. It can feel quite "scary" - this might just be me coming from a village, but even in other major cities I don't feel quite as fearful of being mugged/sexually harassed or assaulted/a terror attack as I do in London!
In all, I love to visit London but I would never live there. If I wanted to live in a major city, which tend to benefit from being diverse, I would definitely either live in Manchester or Edinburgh - the former of which is slightly better connected but much less pretty, the latter of which is beautiful but also expensive, though no where near as much as London.