I was mainly thinking about Project Thor, which roughly means that going at mach 10 (~3km/s) is like being made of TNT energy-wise. Now, current space-shuttles and the ISS weigh around 100 tons, and I'd imagine being able to get at least 10km/s, if not 30 with asteroid mining-level space tech, which should bring spaceships into the kiloton TNT range, that while far from a hydrogen bomb, packs the punch of a smallish fission nuke. So, while it probably won't be easy to wipe out big cities, immense damage is guaranteed.
What I can't estimate properly due to insufficient knowledge is the atmosphere's ability at stopping/limiting such threats, for all I know spaceships/rocks going at too steep an angle might blow up very high up, while stuff going at a gradual entry might be significantly slowed, Although as a rule, bigger things should care less about the atmosphere.
Edit: CellBioGuy's comment points out that spaceships aren't (and probably won't be) built to withstand reentry at dangerous velocities, making at least spaceship-jacks less of a threat.
A reality of physics, and one that doesn't get much play in science fiction, is that as soon as humanity gains space travel, anyone in the asteroid mining or space travel business will have city-busting capabilities at their fingertips.
It's there in classic sci-fi, but not so much recently.
This discussion was started in the comments to:
http://lesswrong.com/lw/gln/a_brief_history_of_ethically_concerned_scientists/
In the "Ethically Concerned Scientists" post, Izeinwinter commented:
A great start to a discussion here.
You've considered people going loons and some general security, but it would then become a hacker war along the lines of who could break the security and gain control of the space ships.
It doesn't address the problem of the leaders using the ships as threat weapons, since they have legitimate control, but can still make terrorist decisions.
And I'm terrified of your idea of turning spaceflight, which I see as the ultimate freedom, along the lines of Niven's Belters, into a state-controlled affair like the Soviet navy with political officers.
Now, one thing I think is a useful safety control that doesn't lead to worse problems is the destruct option. All major rockets have them right now, since if it goes out of control it's a huge hazard for a great distance. And although I don't like the idea of all personal spaceships being under a safety officers thumb, it might be better than the alternative of terrorist groups gaining control of asteroid mines and holding the world hostage.
You're right about great story potential though, in any of these scenarios.