Now, the left wing was struck by debris during the launch, thought to be foam insulation from the external tank, and it was not thought to be a problem. Obviously they're going to be looking at this a lot closer.
Columbia was the heaviest of the shuttles, and because of that it's never been used for a space station mission, it's too tough to get it into the 56 degree inclination orbit the station is in (that orbit was a compromise to make it easier for the Russians to get to the station, it was originally supposed to be only 26 degrees). Had it gone to the station, it's almost certain that any external damage would have been noticed, and they could have stayed at the station until it was figured out what to do.
Columbia was carrying a very heavy load of science experiments, and the crew had been working in two shifts around the clock for sixteen days. It's possible the experiment load might have had something to do with the accident as well.
It's been pointed out that the space station needs regular reboosts of its orbit from the shuttle, and the Russian re-supply flight can't manage this. I don't know yet how long shuttle flights could be grounded before this becomes a serious danger but it's a major consideration.