nah, the bad areas are not dangerous, they are just rough. which means, you are more likely to get heat if you live there (law of the jungle, people trying to be the boss of the street n' all that crap) rather than if you are a tourist. pickpockets are rare (but not inexistent, more likely to run into them around westminster + touristy areas) and there's plenty of police.
i mean, i would not recommend it as a holiday place. it's not that great to visit, it's quite expensive, clubbing / pubs can rape your wallet. the public transport is confusing and packed 90% of the time, it rains quite a lot, there's pigeon shit everywhere, and you'll meet more polish people than brits.
(breath)
the food is shit. the pubs are mostly inferior to what you find elsewhere in europe, and there's basically 3 places to see, camden market (which is really cool, granted), madame tussauds (the wax statue place), and the trocadero (which is basically a shopping center).
most londoners, when they want to sightsee, they go to the mall - im not joking.
sure they got downing street and big ben, but come on, EVERY city in the world has something old to see, and many are far nicer. even Edinburgh is nicer, the architecture is everywhere, not just in the central part, and people actually live there. Central london is all offices, or very expensive shops which all sell the same stuff.
if you live in the US, you'll laugh at what we try to pass for skyscrapers here. the architecture of London is dull and grey, more so with the constant rainy weather. And to make it all worse, there's the scum.
As soon as you step out of westminster (the center of town) you'll run into rat-eaten stalls of the cheapest stuff that can be manufactured in china, sold by people who come from all the worst parts of the world.
I would say, just about every city in europe is better than London to visit - Amsterdam, Brussels, Paris, Rome, Barcellona, Madrid, Lisbona ...
If anything, try going to Manchester, it has most of the issues that plague London, but at least the nightlife is decent AND you can afford it. Or Dublin. Or Edinburgh.
Before you think England .. remember this video:
https://www.youtube.com/watch?v=YR5ApYxkU-U