The Export dialog box has a Cancel button that only works when the object you're exporting is very small, which is to say, it's not there for you when you need it most. If you go to export something very large (on purpose or by accident) but then change your mind when you see how slowly the progress bar is moving, you're stuck. You have to wait it out, unless you want to turn off the computer.

Am I the only one experiencing this? I use 2 different iMacs, it happens on both of them.

It's a common problem, and I think was supposed to be fixed for the next major version. Just tried it out, though, and I get
1. A see-through dialog with 'Export in progress'
2. After a while, a non-reactive Cancel button.

So maybe it makes sense to remind devs of this not working (yet).

You can browse the bug reports at https://bugs.launchpad.net/inkscape to find the previous report(s).

Thanks for the link, I forgot about that site.