I've read this thread with considerable interest, and am with Ross and the Catmeister on this one.
Born and raised in Madchester and then semi-rural Cheshire, I've been in the Smoke for about 20 years now. On the rare occasions I drive into the centre I always curse the place: as the largest city in Europe I guess one should expect a little congestion. So avoid, if possible. If you can't, get in first thing, or late, or on Sunday.
Seriously, compared with most places (Manchester, for example) public transport here is utterly fantastic. For all that we moan about the tube, it's generally fast, regular, and gets you where you need to be. Buses all over the place - 24 hours! More taxis then you can shake a stick at. Overground trains picking up the rest of it. And Ross and Catster have summarised the myriad cultural reasons to stick around better than I ever could.
Why else? I'm part of a mixed-race family. Although we attract the odd look or comment, it's rare enough to be noticed. Travel 30 miles in any direction from this polyglot melting-pot and I can guarantee there will be stares, muttered comments, and worse. Hard to think of anywhere on Earth that would be more relaxed about miscegenation. New York maybe? Amsterdam? That's about it.
There are plenty of green spaces to be found if that's your bag. I'm lucky enough to live right on the edge, so there's green belt out back. The trade off is a long commute. But that gives space for a seat, and time to read, think, listen to music.
All in all I wouldn't change it for anything less than a colossal Schloss in a ski resort. But it's a sh!t place to drive.