Topography and bicycle culture
I've been riding all day in Seattle's most urban environments, and found them to be pretty good for bicycling. We have a few bike lanes now, and the culture is definitely very bike-friendly, so it's certainly not a threatening place to ride. There are, however, gigantic hills everywhere you turn. Personally, I like the hills, but I can imagine that they pose a formidable obstacle to some would-be riders. Contrast this to places like Amsterdam and Copenhagen, both of which have enviable bicycle cultures, and are also flat as a table top. I wonder: would these cities be the bicycle Meccas they are if they weren't so level?