Generally it only takes one driver wanting to stick to the limit for everyone else to do so too, because overtaking is contraindicated. Which isn't to say that it never happens :(.

I live in Edinburgh, which has been gradually introducing 20mph limits over the last couple of years, and I've actually taken to them much better than I'd expected to: they don't reduce my average speed all that much, especially as the arterial roads are still 30mph, and they're much more pleasant when I'm out of the car.