It's too hot. And going to get hotter. While Shiver linked to another "wear sunscreen or die" article.

When thinking about space colonies, one of my big reasons was not "destiny" or "spread our eggs" or "flee the brown folks", but "whee, controllable outdoors climate, including gravity". People worry about radiation in space but it sounded like if you piled up the outer shell -- 10m thick, maybe? -- you could even get better shielding than the atmosphere provides. And no UV worries.

These days I'm more likely to think of arcologies, or paraterraforming the Earth, though still, somehow a km-wide space cylinder doesn't upset the imagination as much as a 1-km high roof. Maybe because of gravity, even though the cylinder would be spinning.

I link again to http://www.kuro5hin.org/story/2002/12/21/17846/757 in which long-thinking machines like unto Culture Minds choose against unshielded poorly maintainable nuclear reactors.