Before I get into it, let me say that the majority of the information below and analysis of it comes from one of my partners in crime (coworker at Nexenta), Kirill (@kdavyd). All credit for figuring out what was causing the long export times, and reproducing the issue internally (it was initially observed at a customer) after we cobbled together enough equipment, and analysis to date, goes to him. I'm just the messenger, here.
Here's the short version, in the form of the output from a test machine (emphasis is mine):
root@gold:/volumes# time zpool export l2test
That was a test pool with a couple of TB of L2ARC, filled up, with an average block size on pool of 4K (a worst case scenario) -- and then simply exported, as shown. If you are presently using a pool with a significant amount of L2ARC (100's of GB's or TB's), (update:) and you have multiple (3+) cache vdevs, and have any desire to ever export your pool that has a time limit attached to it before you need to be importing it again to maintain service availability, you should read on. Otherwise this doesn't really affect you. In either case, it should be fixable, long-term, so probably not a panic-worthy piece of information for most people. :)